I cannot explain why it was not possible to delete the 20 or 21 versions, or the original dmg files, but the core issue that I have seen often is the generation of the 21 labelled version. It is easy to reproduce and confusing but not critical. I believe I have worked out what is going on.
I can generate the effect reproducibly by simply clicking on 'keep both' in the dialogue box that appears when you generate the volume from the dmg file and then are asked to drag the icon into the Applications Folder. The mac then detects the existing version with the same filename (for example Media Center 20.app) and asks what you want to do. If you are simply upgrading, you replace the older version with the update and all is fine. If you are feeling cautious and want to keep both versions you click on 'keep both' and the mac installs the new one alongside the old one.
Normal behaviour when this happens is the mac relabels the newer version with an additional -1 in the filename or adds the word 'copy'. I often see this when I accidentally add two copies of a download into my download folder. The second time you do it, the number increments to -2, and so on. What is happening here is that the mac is misinterpreting the number at the end of the file name 'Media Center 20' as the copy version of the app file. When you add a further version in, say, the 20 series (the current one) the mac increments the 20 to 21 to avoid confusion as it has the same filename.
If you store two copies of the identical app version such as 20.00.57, they both work fine and you can select either but one has been renamed 21 in the filename. You can delete either at will and rename the 21 file to 20, if you wish - though there is no point in having two versions of the same update. The problem arises when you wish to keep a slightly older update along with the latest update from within the same series: the filename remains at the series number but the file is not the same. The easiest way to see which update you have is to look at the version number on the file information.
It would be simpler if the filename incremented with the version number or if the filename could be constructed so the mac did not interpret it as the copy number and just added a helpful -1, -2 etc. In the meantime just select the versions you want and rename the files to something that works better for you.
For completeness, I used to see this on Series 18 and 19 as well.
Its never dull with MC!
MikeBRG iMac; OsX 10.8.5; MC20 for Mac right up to 20.00.57
