When I create new macro, hot keys I have already assigned are still displayed as available for use.

This could be one of two things.

1.  The hot keys will still show as available if they are assigned with Program Specific or Window Specific scopes. The same hot keys can be re-used with other Window or Program Specific macros. If a macro is assigned with a global scope, then the hot key will not be available.
   
   
2.  The other possibility would be if you are using two different macro files.