Just can't imagine why the Sten mags didn't fit the Sterling...

Jim, I explained why in my Earlier post Buddy.

without a special adaptor, the Sterling mags will NOT fit directly into a Sten!..
This was a cunning built in design by Sterling when Marketing these SMGs. As if you had an Army that used the Sten, you would have to replace the entire 'Fleet' to use the VERY reliable Sterling Magazines! That meant, you could NOT get away with the cheaper expedient of purchasing just Magazines. To upgrade the reliability factor of your Sten 'Fleet'!