We’ve discussed in the past (with respect to making early Shabbos) the machlokes between R’ Yehudah and the Chachamim regarding the proper time for mincha and ma’ariv. According to R’ Yehudah, the latest time for mincha is plag (1 1/4 hours before sundown), with ma’ariv permissable thereafter; according to the Chachamim the latest time for mincha is shekiya, sundown, and only then can you daven ma’ariv . The crux of the issue is what time the korban tamid bein ha’arbayin, which the tefilah of mincha corresponds to, was offered.
The gemara does not come to a conclusion as to who is right – d’aved k’mar avid, d’avid k’mar avid – one is free to follow either view so long as one is consistent. In other words, you can’t wait until after plag to daven mincha in accordance with the view of the Chachamim but daven ma’ariv on the same day before shekiya in accordance with R’ Yehudah’s leniency.
At first glance the reason why either view is acceptable is because sfeika derabbanan l’kula. When dealing with a mitzvah or issur derabbanan, where there is a doubt as to what the halacha is, one is permitted to be lenient. Since tefilah is derabbanan and there is no clear resolution who is right, one can be lenient and wait until shekiya to daven mincha or daven ma’ariv after plag if one davened an early mincha.
However, that’s not how the Rambam seems to see it. Hil Tefilah ch 3:
ויש לו להתפלל תפילת ערבית של לילי שבת, בערב שבת קודם שתשקע השמש; וכן מתפלל ערבית של מוצאי שבת, בשבת: לפי שתפילת ערבית רשות, אין מדקדקין בזמנה
The Rambam does not invoke the rule of sfeika derabbanan l’kula to justify davening ma’ariv early, but instead invokes the rule of “tefilas arvis reshus,” i.e. ma’ariv is less of an obligation than other tefilos because it does not correspond to a korban.
Another indication that this is not a real safeik is the Rambam’s ruling (Hil Temidim 1:3) that the tamid shel ben ha’arbayim was offered until sundown:
תמיד של בין הערביים, שוחטין אותו משיאריך הצל ויוכר לכול שהאריך, והוא משש שעות ומחצה ומעלה, עד סוף היום
Were there any real doubt as to who is right – R’ Yehudah or the Chachamim –the rule of sfeika d’oraysa l’chumra should apply and we should avoid offering the korban after plag. Yet, the Rambam allows until sundown for the korban to be brought.
Apparently the Rambam understood the gemara's conclusion not as a statement of doubt about who is right, but as an affirmation that both R' Yehudah and the Chachamim are right. Both positions are tenable halachic approaches. Since the Chachamim are the majority view we give preference to their opinion, but since tefilas arvis reshus one rely on R' Yehudah.