Isn't this pretty obvious? The input string is processed from left to right and the regex first processes values in parentheses.
This means (is) is first matched and then (Ga), resulting in your echo (and only returns it once since you use /t).
For the second example the entire string between parentheses is matched, this giving you that particular echo.
As for the last question, pretty clear to me...
\0 = how many matches were found
\n = what number a given match was (e.g. the 6th match found)
\t = the text that was matched by the regex (/n in text form)
\a = all of \t in one string
\A = \a without spacing out the individual matches
