why is that happening and which are those 44 strings?
It is even more messy. On download.moodle.org, total number of strings in standard components is currently shown as 26154. But if you extract the en.zip you will actually find 26185 strings. And AMOS translator itself reports total of 26110 strings.
One particular reason for these differences are strings with the "_link" suffix in their id. They have special meaning when displaying a help popup and they should not be translated. So AMOS does not show them (and does not count them into those 26110 standard strings available for editing) while they are still present in the generated language packs.
Another case coming to my mind are certain empty strings in TinyMCE plugins. There was some special care needed related to not 100% accurate translation stats and certain corrections were applied. Details can be found at https://lang.moodle.org/mod/forum/discuss.php?d=3372.
I admit it would deserve some cleanup. AMOS has been continuously developed for years and various features were added. When some of them affected how strings were counted in a particular part of the processing-chain (e.g. in the translator UI or in the ZIP generation tasks), chances are that we did not properly update all other places. It is a clear bug for sure. And should be fixed. Eventually.