I'd guess it's because areg reports transactions, while reg reports postings. That sounds ok to me.
The situation is unclear though:
says "Each aregister line item shows: • the transaction's date (or the relevant posting's date if different, see below)"
says "Transactions whose date is outside the report period can still be shown, if they have a posting to this account dated inside the report period. (And in this case it's the posting date that is shown.)"
3. The aregister command contains this comment: "TODO: need to also pass the queries so we can choose which date to render - move them into the report ?"
I'm not seeing the behaviour described by 2. Even if I declare the secondary date on the transaction rather than the posting, --date2 doesn't display it match on it.
So I'd welcome a bug report or PR, changing either the docs or the behaviour, for further testing. There may be some deeper implementation/design/usability reasons, but I don't remember them.