The difference:

Shown are 6.5X54 - M1903 (also M1900, Y1903 and variants)
Note how magazine spool 'cradles' cartridge of proper profile.

It's about bullet profile, overall cartridge length, and clearance.

The Schoenauer rotary magazines, particularly those of pre M1924 models that lack a 'guide ring' near the cartridge shoulder, are rather particular in regards to overall cartridge length and profile of entire cartridge.

The surfaces of rotor and surrounding magazine well are precision machined to accept cartridges of exact profile specified as the proprietary cartridge for the specific model of MS (9.5X57 for M1910...). Proper fitting cartridges thus fit as would proper fitting roller bearings.

If the bullet profile is too short or narrow, particularly about 1/3 back from tip of Eley specified projectile (drawing in above post), the cartridge may shift and / or the tip fall partially into a void causing feed jam. If too long or wide they simply will not feed.

It should be noted, however, that when fed with proper fitting cartridges the Schoenauer Magazine operates with the amazing smoothness and reliability that made them legend.