Because "some" of the bullet manufactures want to keep the chamber pressure down in all the older 30-30 rifles still being used. My Hornady 170 grain flat point 30-30 bullets are .308 behind the crimping groove and .300 forward of the crimping groove. This again is a way of lowering bullet resistance and chamber pressure.
In the Speer reloading manual No.9 dated 1974 they use a .308 Plinker 3/4 jacket for the .303 British which is .312 diameter and all the other Speer .303 British bullets are .311. When the bullet has a softer lead core when fired the bullet will expand outward to fill the bore.
Many of the older calibers are loaded to lower pressures by the factory than they are rated for, this is for the same reason. I also have a Savage/Stevens 200 rifle in .223 that has a longer throat than my AR15 rifles do to reduce pressure and keep people from damaging commercial rifles firing military ammunition.
The rifle and ammunition manufactures make the chambers on their rifles big enough to fit the company lawyer into and the bullets small enough to squeeze past the lawyer.