Tillering and limb straightness question

[NBK]

Wanted to ask this on this forum because I figured guys who actually build bows would know best.
I have several custom longbows from very well known bowyers both r/d and straight limbed and when I sight down the belly side of the bow the strings track down the middle of the limbs but near the tips I see that there is a slight "twist".  
Is this a normal phenomenon?  
Bows shoot fine and like I said the string tracks true but out of all my bows (a bakers dozen) only one has perfectly straight limbs the entire length.
I have to assume that due to this appearing on multiple bows from multiple bowyers that this must be a normal occurrence when tillering.  (I admit that I'm the kind of guy who cannot sweep anything under the rug!)
So for you gents who actually build bows I'd appreciate your insight.  
Thank you for your time.

[monterey]

Check to see if the string loops are too small.   Very tight loops can torque the tips a bit.

Other than that, I'd just live with it if they shoot okay.

[Sixby]

Different levels of bow building. I do not really see how a tip can be off and the string track down the center of the limb.
God bless, Steve

[NBK]

String loops are fine and it's been a few bows with various strings.  
Sixby:  It's hard to explain without actually seeing it (and it is slight, nothing grossly out of whack) but when looking straight down the limb, if the area above the fades were parallel with the horizon you can see that nearer the tip it's rotated a bit.  These are narrower limbed, deeper cored longbows so the string still appears to be in the middle... (kinda like there's no edge to a circle).
I'm not unhappy with the bows.  I just thought that this is perhaps a normal side effect of the tillering process and I'd learn something for future reference.  
Thanks for the replies.

[Wolftrail]

Sounds like the tips are out of whack or maybe the nock grooves are not equal.  Does it have prop twist..? then it may be possible that the string tracks center. Better it you had a picture.

[Carson81]

Like Wolftrail pointed out, if you have offset and equal limb twist in each limb, the string can still track true and the bow will shoot just fine. Limb twist like you describe occurs in selfbows when one side of the limb is slighlty thicker than the other, and it doesnt take much of a difference. I suppose with glass bows it could also be glued in. Can you detect twist in the limbs when unstrung?