I have a layout system that keeps the bow limbs very even side to side but still get a little string drift occasionally. With everything perfectly even, I suspect wood density changes through the limb causes the drift.
I bought a contour gage from Lowe's to check the symmetry of my limbs, works really well.
I have found that uneven wood side to side in the fades and handle will drift the string more than you would suspect. I use my gage to help me get everything perfect.
I have had little luck moving the string with strong side wood removal on most of my bows with bamboo backed bows being the exception.
When all else fails, out comes my heat gun, it works every time.