It's simply the luck of the draw. I have two English and one Australian Lee-Enfields that can happily interchange fired cases. Another, made in Chicopee Falls, MA, will not.

I've had similar results in my Krags... although I've never really paid much attention to it.  I usually keep my brass segregated, especially since I neck size only.  However, since I now shoot cast out of my sporter, I have 200 rounds of brass designated just for that rifle (I expand the necks with a .303 ball, and I taper ream the case necks so the brass doesn't shave the lead as I seat the bullets).  I just always assumed that since the chambers were cut using a reamer, they should all have pretty much the same dimensions.... but I'm also naive that way, LOL

FWIW-Further update: I've now tried 20 'fired' Winchester .30-40 cartridge cases in eleven different Krags. These rounds were fired two days ago in two different model 1898 Krags. The cases have not been resized, since last firing.

I purchased these cases as new unprimed brass. They have been loaded and fired, at least three to four times, in five different U.S. Krags. Prior to each loading, they were only neck-sized with an RCBS F.L. .30-40 Die, backed-off two turns.

All 20 'fired' cases easily chamber in ten of the Krags, ranging from a model 1898 carbine #119744 (1898) to a model 1898 rifle #470165 (1903). Two of the Krags are model 1899 carbines.

The only Krag that would not accept these 'fired' cases is a 'sporter' that was re-barreled by Sedgely & Company. Its barrel is a re-cycled 1903 Springfield barrel that is dated 1905. Originally, the barrel was chambered for the .30-03 cartridge. Sedgely shortened this barrel at the breech and re-chambered it for .30-40.

The 'Sedgely chamber' will accept factory ammo and F.L. Re-sized cases, but, its chamber is tighter than the chambers on my original Krag barrels. It will not accept the 'fired' cases from the military chambers (without F.L. resizing).