That the Almagest was not (recorded as having been) translated into Latin prior to 12th century does not rule out the study of the motions of the stars (both fixed and wandering) having been important to the Cult of Mercury and preserved by the Mercurian-descended magical traditions. The rules-stated importance of "astrological calculations" in Hermetic ritual magic (which is derived from Mercurian magic) would be pretty strong evidence of this.
This would produce a Hermetic-specific tradition of astronomy/astrology, inherited from the Cult of Mercury, independent of the mundane Latin-speaking world losing Ptolemy with the end of Greek education in what was the Western Roman Empire. While presumably it would be related to classical/Hellenistic astronomy/astrology, the Mercurian-Hermetic system could have any number of differences accumulating over several centuries of practice.
That "could" allows for a possibility that 767 AD was 905 or 950 years into an Age of Aries named according to a system (otherwise undetailed) used by Bonisagus that doesn't name ages with any reference to where the Sun was on the vernal equinox at all. Of course, then we don't know when the "third day of Pisces" is, either, since it would also be according to a Mercurian-Hermetic system we know very little about.
Alternatively, it is possible that Mythic Europe has its constellations shifted 30 degrees from the real world -- I think 4th edition's The Mysteries suggested that as one of its resolutions? In which case the interesting thing is Mythic Europe's mundane astrologers adjusted their tropical zodiac to switch the start of the year from Taurus to Aries when the vernal equinox moved from Taurus to Aries, instead of sticking to traditional definitions.
I personally prefer a purely Doylist resolution ("Pisces and Aries were reversed in the Oath by the authors of 2e, we should just swap them back and revise references to the Age as necessary"), but, eh.