Well, I can tell you that a seatpost that is slightly too small will cause you to be able to clamp the seatpost in, but allow just enough give so that no matter how tight you try to get it, the seat will still rotate from side to side while being ridden & pedaling.My fixed gear build has a bugger of a seatpost/tube size(mine also came sans seatpost), and I had 2 bike shops measure it. Both confirmed it measures a 25.5-6(is a bit oblong in a spot) but sold me a 25.4 saying it would work fine.
I have to clamp it just right, and even then, sometimes I have to have my hubby rewrench it tighter to work reliably. I think the most frustrating part is that .1 is too small to make a shim for. Only thing I have sortof gotten to work is a layer of electrical tape wrapped around once, with a slight overlap in the oblong area.