Hi Lauren,
While Gordon is right about the aspect ratio, also for the same size there are always differences in diameter between the various brands.
Furthermore I have seen a tyre sidewall marked "11.00-20 replaces 10.50-20". I have since learned this was a "new alternative size marking" for the 10.50-20 tyres listed in the wartime Data Book of Wheeled Vehicles, so it must have been a wartime change. (Interestingly, while this tyre originally had Dunlop TrakGrip tread, it was recapped at least twice, the last time with NDCC bar tread.)
If I were you I would try to exchange your 10.50's for 11.00-20 tyres.
Hanno