exec() is returning only the set of captures for the first match, not the set of matches as you expect. So what you’re really seeing is $0 (the entire match, “a”) and $1 (the first capture)–i.e. an array of length 2. exec() meanwhile is designed so that you can call it again to get the captures for the next match. From MDN:
If your regular expression uses the “g” flag, you can use the exec method multiple times to find successive matches in the same string. When you do so, the search starts at the substring of str specified by the regular expression’s lastIndex property (test will also advance the lastIndex property).