As @ohrenkino wrote:
Example situation:
You have this tracks in your album folder called \SuperAlbum:
01 - First Track
02 - Second Track
04 - Fourth Track
You could then count the tracks per album and say:
The last track is 04 so therefore 4 tracks should exists. But the count only give you 3 tracks. One track must be missing.
Why this doesn't work (for the same folder \SuperAlbum):
01 - First Track
02 - Second Track
03 - Third Track
You would count 3 tracks in this folder, where the last track says 03. There is no way to see/count that the 4th track is missing.
There could be even a 5th and more tracks missing.