American Express does not backdate authorised users anymore, so it may be reporting the month she added you as the open date. New accounts hurt scores, so that could be the issue. If she has a balance on it or derogatory information, that would also cause the score to be lowered.
So my fiancee added me to her AMEX as an AU. Before the card reported I had a FICO TU score of 640. After reporting it dropped to 617 at this same time my EQ jumped from 619 to 673... Can someone please help me understand this a little bit better...
To answer your topic question...theoretically, yes.
When you are added as a AU or ACM for AMEX, you "inherit" their credit report to a certain extent. Depending on what her score and report were at the time you were added, this could impact your score. This "inheritance" factor is a double edged sword. While it can benefit the AU/ACM if they have no credit/limited credit and the account holder has pristine credit, the opposite is also true; if the cardholder's credit stinks, then the AU/ACM will not see a benefit.
Also, keep in mind that, if you're getting your TU FICO score from here, it's a TU98 scoring model, a very old model. More recent TU models are 04 and 08, 04 being the more common. EQ's Beacon 5.0 here has always been a newer model than the TU98, though different, of course. That could explain the wild variations in scores.