Prexies On Cover

Here are some postally used Registered Mail covers with Prexie stamps from 1948, 1949 and 1951. My question is why the variable rates? Were there interim postal increases? Or perhaps some have requested Return Receipts and others have not. (BTW, the addressee name/address has intentionally been erased for privacy).

Here's a 27c rate from April 1948 (Registered, Return Receipt):



And a 23c rate from April 1948 (Registered, no Return Receipt). Does this suggest that the April 1948 Return Receipt charge was 4c?



Now fast forward to October 1949 and a 28c rate is shown (Registred, no Return Receipt). I assume this had to do with a postal rate increase:



And finally a September 1951 cover with a 33c rate paid (Registered, Return Receipt). Could the Return Receipt have cost an extra 5c at that point?: