I would guess those doing the ~$20 rental are a mix of people who'd normally see the movie in the theater and those who'd normally wait for the ~$5 rental. That cannibalizes both release segments.
I can't see that many adults renting a movie they've already seen in the theater and believe...
Are you sure it works that way? I've not read the fine print but would expect that rental to be once over a 48 hour period. That is, you have up to 48 hours to start the stream for a single viewing.