Sequences 'a' and '1' are unambiguously interpreted as abbreviating '<Keypress-a>' and '<Keypress-1>', and similarly for any other printable ascii char.  '<a>' is interpreted also as '<Keypress-a>', but there is no reason to add the brackets.  In any case, binding a specific printable key is fairly rare.  '<1>' is ambiguous is that it could also mean '<Button-1>'.  Since '1' is available to abbreviate '<Keypress-1>', tcl currently chooses the button interpretation, and has AFAIK for at least a decade or more.  In any case, what tcl does is not a Python bug, and it does not matter what it might have done in old versions.

Note.  Buttons 4 and 5 are used on Linux for a mousewheel.  On Windows, they correspond to actual buttons, if present.  I just tested with a mouse with two side buttons.  '<6>', etc, is seen as key-6.