Mike Irwin, the .270 Winchester (or 6.9x64mm) was developed by Winchester Repeating Arms Company in 1923 and unveiled in 1925 as a chambering for their bolt-action Model 54. 1923's twenty years later than 1903, so I doubt that old military cartridge was used as the basis for the .270.
Many cartridges "reformed" from parent ones will have a different case length than their parent case does. .243 and .270 Win are just a couple. Especially if the parent case is just sized down in the neck, it'll often be a few thousandths longer.
US Navy Distinguished Marksman Badge 153
Former USA Palma Team Member
NRA High Power Long Range High Master
NRA Smallbore Prone Master