Ours works by calculating where to store the Differential data (c/o the variable you use) and flushing that location before re-use if populated (note that it doesn't flush if performing a Rescan). This flushing is optional, but if used (as recommended, for reasons we trust are obvious), it does rely on your 'cycle' (manual or automated) pointing at a folder with files in (if you want them flushed).
For example, if you started it off on day-1, using %DAYOFMONTH%, and ran it for Differential backups on days 4/11/18/25, it would create those folders that month. If, in the next month, you did a full backup on day-1 (again), but this time did your manually-triggered runs on days 5/12/19/26, it wouldn't flush folders 4/11/18/25 from last month - because in the 2nd month, the days you chose to run it don't point the profile at those older folders, but at folders 5/12/19/26
There's no concept of 'numbering' or 'how many' except by implication (the software doesn't maintain a count, or even a record of where it backed up to recently). Thus, there's no code to 'delete all but the last X'. The logic works on picking a cycle and sticking to it. It will work if you stick to 4/11/18/25 or 5/12/19/26 or whatever, but not if you keep changing the days (unless you manually delete those older diff folders, or 'cope' with them still being there, probably populated with old copies)
For example, if you started it off on day-1, using %DAYOFMONTH%, and ran it for Differential backups on days 4/11/18/25, it would create those folders that month. If, in the next month, you did a full backup on day-1 (again), but this time did your manually-triggered runs on days 5/12/19/26, it wouldn't flush folders 4/11/18/25 from last month - because in the 2nd month, the days you chose to run it don't point the profile at those older folders, but at folders 5/12/19/26
There's no concept of 'numbering' or 'how many' except by implication (the software doesn't maintain a count, or even a record of where it backed up to recently). Thus, there's no code to 'delete all but the last X'. The logic works on picking a cycle and sticking to it. It will work if you stick to 4/11/18/25 or 5/12/19/26 or whatever, but not if you keep changing the days (unless you manually delete those older diff folders, or 'cope' with them still being there, probably populated with old copies)