Some versions just have too many bugs to be fun to use.
It just happens when new features get into a release. That's why it was decided that only the alpha testers should get to know about the filename of some releases and thus they are diclosed in the announcements to the muttdev mailing list. But if you are a subscriber to muttdev then you will probably get to know the filename. A new version of MUTT is usually rleased every Friday to Saturday night. Since July we have seen one release per week.