Update docs/using_eessi/setting_up_environment.md

Co-authored-by: Thomas Röblitz <[email protected]>

docs/using_eessi/setting_up_environment.md

@@ -9,7 +9,7 @@ There are a few different scenarios where you may want to set up the EESSI envir
 1.  You are already using Lmod with version >= 8.6
 
     In this case, we recommend unsetting the `MODULEPATH`, because EESSI is not designed to mix modules coming from EESSI and from your system.
-    since they are effectively two different OSes (though this may not be possible at some sites, and some compromise may be necessary):
+    The underlying reason is that very likely EESSI and your system are based on two different operating system distributions - EESSI uses Gentoo, your system likely uses some other distribution. Note, unsetting `MODULEPATH` may not be possible at some sites, and some compromise may be necessary:
 
     ``` { .bash .copy }
     unset MODULEPATH