There has to be a small unused area at the start for the partition table. 2MB is far more than is needed for that, but leaving less would tank performance due to the fs blocks and disk blocks not being aligned.

I know Gentoo has it masked for testing with this note, which is probably the same reason why other distros without the same mechanism don’t have it at all:

Testing. An upgrade from 102 isn’t recommended due to downgrading most likely not being possible. Back up your profile before attempting. Fresh install should be fine. Bug #910229