Powder vs. bullet stability.

[mak91]

Ok, this has me stumped. First off I am no reloader expert but have been tinkering with different rounds for years. I've been playing with subsonic loads for a 11 inch ar57(5.7x28) upper that I have. I am using lc 55 gr. FMJ bullets. Since I had some Trail Boss I worked up to 2.5 gr. I had my chrony about 10 ft from the muzzle and a cardboard target about 1 ft behind it. Speed was 925 - 950fps at 2.5 gr and the bullets were going sideways through the target. I switched to SR4759 and worked up to 4.5 gr, got to a speed of 950 -1000 fps and perfect little holes in the target, no keyholes. The barrel has a 8.5 twist and according to the Miller stability formula this bullet should stabilize in this barrel down below 500 fps. I can't believe the difference in speed between these two loads could have made the difference in stability. The oal of the rounds were exactly the same and both loads cycle the gun fine. I understand that the burn rate of the two powders is quite different but don't understand how that could cause this. I thought bullet stability was pretty much barrel twist, bullet length, bullet weight, bullet speed. Can anyone explain to me what might be going on or what I might look for as a reason?

Thanks.