The 12mo/12000 mile "recommendation" is a CYOA statement, doesn't have anything to do with the mechanical interactions of the bearing, grease, axle, etc... Same as the old 3mo/3000 mile "recommendation" for engine oil.
It is interesting that Dexter also makes the Never Lube bearings that typically don't need annual repacking. And if you look at the internals there is not that much different between the EZ Lube and the Never Lube. My bearing repack schedule is every five years or so depending on how many miles I rack up.