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Why is it that many guitars tend to have more relief on the treble side? And very few the other way around. Or is it just my eye?

My guitars do it too. If I always try to plane the fingerboard as straight as possible, while joined to the body. Everything looks good, but under tension and neck rod tightened, the treble side has a bow to it. Does it have something to do with string tension? Using light strings, one has 83.81 lbs from the E, a and d strings, and 76.73 lbs from the g, b and e strings. Thats only a 7 lbs difference, and the heavier side being the bass side. Wouldn't that make the bass side bow more? Is this the point where I should get a string tension jig? Does anyone else notice this too and have any insight?

Could be a quirk in how you are measuring or observing relief, but if we assume your observations to be accurate then it would likely just be chance. Of course if the phenomenon were consistently observed on necks from a single maker, it would likely reflect a flaw in their tooling or processes. Even made well though, wood can move, and more often than not necks can develop differences between the bass and treble sides, sometimes so slight as to be insignificant, other times not so slight.

Ideally the opposite would be preferred - slightly more relief on the bass side than on the treble. This is how we intentionally level boards and frets in our shop, and how we teach it in our classes. Left to their own whim though, necks can have a 50/50 chance of going either way if they move, and it really couldn't be attributed to the difference in string tension.

As you suggested though, if your observations seem to be at unreasonable odds with random chance, observed across many different makers, and you're not confident your appraisal is entirely accurate, then it could be a quirk in how you're making your observations.

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Eschew obfuscation, espouse elucidation.

Once I had a very poorly working truss rod system on a fret dress and the neck would sort of follow the torque on the rod. The neck would wind up a little when the rod was turned and hold a little of the twist.

I don't know if anyone else has seen this, and it's only happened badly once or twice, but if it was leveled in a twisted position it could distort measured neck relief differences treb to bass.

I see more relief on the treble side every once in a while but I wouldn't describe it as often.

On my new builds there is consistently more relief on the bass side so I do not have an answer. How do you level your fretboard and frets?

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It's not what you don't know that hurts you, it's what you do know that's wrong.

I don't want to go into details in so much as this is something that really has to be seen to understand what I am describing but manipulating relief is one of the most basic elements of fret work and doing fret dresses.

What I will say is relief can be corrected regardless of how the luck of the draw worked out for some folks with a quality fret dress. It's even better to build the proper relief into the neck prior to ever getting out the frets because imparting proper relief where we want it to the fret board saves fret height for normal wear and tear.

Both methods are things that our students learn in our classes and many of them go back home and get out previous builds and revisit the fret work often correcting relief on their previous builds. That's what I did too a decade ago after learning the methods from Mr. Collins....

I don't know for sure but I strongly suspect that PLEK programming can address correcting or creating proper relief too. Since PLEKs work under string tension it would be easy to program the stinkin thing to do proper relief if the proprietary, canned programs permit customization.

A related topic is compression fretting. With compression fretting (Martin pre-truss rod necks) we are looking for relief to be set on both the treble and bass sides as well just like with any other instrument. Only difference is the consideration of string tension again and imparting more relief on the bass side.

I frequently see instruments with more relief on the treble side and often when explaining what's not perfect to a client about their instrument call it luck of the draw since most f*ctories make no effort to address proper relief. I said most.... not all.....

The big point that I want to make here is that relief need never be luck of the draw. It's something that we can have complete control over where it is, how much there is, more on the bass side, etc. It's also not difficult to do but has to be described, at least for me in the far greater context of the entire fret plane and all things that interact with same.