I understand what happened in general. After you slid left one from the 7th to the 6th, you had 5 days off fall under your vacation. Those 5 days had to have corresponding R-days removed. If I understand your original sked correctly, they just counted the 4 buffer days already removed and took the 15th off as well.

What I don't understand is why they moved the Apr 30th single R-day left of your first buffer day to later in the month (i.e. what part of the CBA allowed that). I was under the impression that any original R-days unaffected by vaca or buffers needed to stay in their original place. Maybe there is something about leaving a single R-day that I missed in the CBA that allows them to move it.