Welcome to the site !!
As far as I know , it is when parts are superseded , in other words , either the part # was changed on some parts , or they made a change to the part that reflects the next year model which may have a better design , but the part should fit either .... there is no real bolt on or functionality issues using either .... but generally , I just choose parts from the first one (single A) , but there should be a asterisk ( * ) on the part # and below the list it should give a explanation as to the change if any ..
Example:
I highlighted in yellow the part with the *
Explanation highlighted
Hope that helped
