Not a stupid question at all, LMG
The original 1903 rifle was chambered for a somewhat less efficient round called, I think, the US Service M1903.
I don't have my reference stuff at hand, but I believe it was a 220 gr RN bullet at around 2300 fps. And the 1903 rifles had a full 24 inch barrel, sporting a spike bayonet.
It soon became apparent that the spitzer style bullet was the wave of the future, and so the .30 US Govt. Model 1906 round came into being. It had a 150 gr bullet at ~2900 fps. This round was loaded in a case just a bit shorter than the 1903 cartridge, which came to be called the .30'03.
There had already been a lot of 1903 rifles produced, and when the changeover in ammo came about, practically all of them were returned for modification. This entailed taking, I believe, two threads off the breech end and rechambering. The result was a barrel about 23.6" in length, and all subsequent new production rifles copied this rather unusual length. Other mods to the rifle included new sights with graduations matched to the new '06 cartridge, and provisions for a detachable blade-type bayonet.
Good sources:
Book of the Springfield by Crossman and
Cartridges of the World by Frank Barnes. Actually, any serious student of the rifle probably ought to own both of these books.
Best,
Johnny
PS - - I see you already found some of the answer. Oh well . . . .