I'm not sure why you are surprised. Are you sure you want all the gory technical details?
Here goes: everytime you change something in the environment (particularly when something is deleted or given a new value) it technically gets fragmented. This is a bit like a database (when you delete records) or a file system (when you delete files). These are marked as deleted, but are still physically there. In the case of a database, the process of "packing" it (or whatever your favourite database program calls getting rid of junk) removes all the junk, speeds up the database, etc. In the case of a filesystem, defragmenting it does the same. Ditto for the router's environment - except that, for Adam2, you have a finite number of possible entries in the environment (80), and Adam2 is not very good of getting rid of junk when the environment is full of trash (i.e., obsolete entries). The PSP bootloader, on the other hand, does a pretty good job of getting rid of trash - so fragmentation hardly ever becomes a problem on routers with the PSP boot loader. As has been said many times, Adam2 is brain-damaged. We have tried to prolong its life, and the defragmentation features of RT v2.91 will go a long way towards eliminating a long-standing Adam2 problem (which the manufacturers never bothered to try to fix, opting to dump it in favour of the PSP bootloader instead).
In all cases, low levels of fragmentation are not a problem. But when fragmentation becomes acute, then problems start to occur - until this is sorted out. So in your case, the fragmentation level would be around 1% - which is not a problem (up to 75% would probably still be okay). I bet you would have no problem upgrading from the web interface now.
The next release of RT will allow you to set a fragmentation threshold, and will have a new tool that will do the defragmentation only when that threshold has been reached. Perhaps I shouldn't let the cat out of the bag, but this feature is already undergoing some serious testing (but of course will only be released when it is ready). In the meantime, it will probably take several months (if not years) before you even need to start worrying about fragmentation levels. But if you are concerned about it, then you can set up a cron job to do it once a month or so.