There are three possible answers that I can think of. One is that bullets tips are not all the same. That is, bullets tips are not always exactly the same and can vary by a few thou if you are measureing off of the tip with a standard caliper. You need to use a stoney point or similar bullet comparator inorder to measure bullets correctly off the ogive.
Two is that your tips of the bullets may be hitting the seater plug rather than the ogive of the bullet hitting the sides of the seater plug like they are supposed to. I have heard that the remady to this is to simply drill it out, but doing this risks creating a burr that will cause runout (I recently experienced that w/ a RCBS die)
THree (which I feel would be a long shot, but possible) is that your press is sloppy or your stroke is inconsistant. Since a seater die is usually kept about .1" above the top of the stroke, you are relying on the stroke stop to make every bullet the same. If this is the case (if you are using an extremely cheap wore out press like a cheap Lee) than you could replace the press or make sure you are applying simalar pressure on each stroke.
Most likely it is the first one, measureing wrong. It could be a combination of the first two.
As far as your COAL goes, the SAAMI spec is just a reccomendation to ensure that it fits into all Magazines basically. The main things you need to know about seating is that seating next to the lands creates excess pressure and you should use caution doing this; consistent seating can make a huge difference in accuracy and POI changes from lot to lot; Many, but not all rifles shoot best seating very close to the lands (a few thou); and lastly that crimping isn't necessary in most calibers.
Hope this helps ;)
I used to re-load but now I "hand-load".
-- Well, at least I try