I found an interesting GR"A quoted in R' Shach's discussion (Avi Ezri, Hil. Talmud Torah) of why women should recite birchas haTorah. The Shulchan Aruch (Y.D. 246:6) quotes the Rambam's statement that women who learn Torah receive reward just as anyone not obligated to perform a mitzvah receives reward if they do so anyway. Had you asked me I would have said the source for this din is R' Chanina's statement quoted in various places (e.g. Kid 31a) that "gadol metzuveh v'oseh", someone who is obligated to do a mitzvah receives greater reward for its performance than someone who is "aino metzuveh", someone who voluntarily performs the mitzvah, but the aino metzuveh does receive some reward.
The GR"A, however, points to a different source. GR"A cites Bava Kama 38 where we learn that even a non-Jew who learns Torah (the 7 mitzvos that apply to non-Jews) can become as great as the kohein gadol.
Why would the GR"A not refer to R' Chanina and instead send you to a gemara that speaks of the reward a non-Jew can receive for talmud Torah? Are we to infer that the reward of an "aino metzuveh" who studies Torah is special and distinct from the reward of an "aino metzuveh" who engages in other mitzvos (R' Chanina's din)?