I'm with you, Benchguy, and I've been doing this for 47 years. Most often I find a variation in OAL when measured from "ogive" to case base. Same bullets; same lot number. I'll use my 6.5 Creedmoor as an example; I have a few reasons that I suspect:
My OAL target with a 140gr Hornady SP is 2.245" base-to-ogive, in MY rifle. It is not unusual to see swings from 2.242 to 2.248, even though most are 2.245. Frustrating!!
1) I don't trim all my rifle cases unless they exceed the maximum length, so does a difference of 0.01" make a difference even though most cases fall in between those two acceptable extremes? Find out: trim all cases every time to the same size.
2) I don't keep a record of the number of times I've reloaded any cases; when the primer goes in too easily, or the neck splits, or the head fractures, I toss that case.
So the potential difference in neck thickness, internal volume, or variable primer pressure may play a role. Find out: always use new cases from the same manufacturer (and even they may vary).
3) A very common and fixable situation is primer seating variation. If I see a longer-than-expected round, I stand it up on the table. If it wobbles, the primer is not seated deeply enough. I gently prime the case again by putting more pressure on that round and that invariably solves the problem. That takes me to:
4) I don't routinely clean the primer pockets unless they are obviously totally black.
Find out: routinely clean the pockets every time.
5) If you buy inexpensive bullets, there may be a difference in bullet base-to-ogive size among those bullets. I found that with a purchase of Remington 130gr PSP that was a bargain sale. I could not get them the seat to the same size more often than usual. I think they were on sale because they were seconds. When I measured the base-to-ogive among a group of 100, there was such a wide variation that I stopped using the bullets. Bench rest target bullets are least likely to have a major variation; hunting rounds even from the major manufacturers might differ slightly. I don't have experience with cast bullets in rifles.
6) I don't use the same primers each time. I may use CCI, Winchester, Remington, and Federal, based on what is available at the time. But I have not really experimented with all loads being the same except for the primers to establish if that affects my accuracy or velocity.
7) Then there's just ME. If every one of my hand loads is exquisitely perfect, am I holding exactly the same sight picture between rounds? Is trigger pressure the same? Am I holding my breath and letting it out slowly, then firing at the end of expiration -each time? Is the rifle in the rest in the same position each time? Did the rest change position between shots due to recoil? Did someone fire a shot precisely before I let mine go off? How about temperature and atmospheric pressure differences between range visits?
8) Just thought of this as an addition: Do you use the same pressure on your caliper when you measure the finished cartridge? I need a lot of luck to do that.
If you hand load just for hunting, putting all of your shots in an 8-inch circle at any given distance should suffice for at least, deer. If you load for competition, that's a whole different world. If you are like me -and maybe most of us here on TFL - your hobby is looking for that "bug hole" or as close to it as you can get. On a good day.