The attached image makes it clearer what you're asking about.
I can confirm on the version I'm running that pressing keys 1-5 will zoom to those levels, but pressing shift with 2-5 does not change zoom..
(I also tried first switching to the zoom tool before pressing the keys - on some other tools pressing shift changes a selection in the palette - but that made no difference.)
I manually changed the shortcut for 1:16 to % and then pressing shift-5 performs that zoom, though that wouldn't be a general solution for 2/3 given different keyboard layouts.
I'd check the
Gimp issue tracker and see if this has been raised as a bug, and if not (and someone running the latest version can confirm it still occurs), raise it as one.