why Rashi gives 2 different reasons for 4 kosos

Rashi on the first Mishna in Arvei Pesachim writes that the 4 kosos on leil ha'seder correspond to the 4 leshonos of geulah mentioned in our parsha:

ארבע כוסות. כנגד ארבעה לשוני גאולה האמורים בגלות מצרים והוצאתי אתכם והצלתי אתכם וגאלתי אתכם ולקחתי אתכם בפרשת וארא:

However, on daf 108, commenting on R' Yehoshua ben Levi's din that even women have to drink 4 kosos because שאף הן היו באותו הנס, Rashi there writes that the kosos correspond to the kosos mentioned in the dream of Sar haMashkim:

ארבע כוסות. שלשה כנגד ג' כוסות שנאמרו בפסוק זה וכוס פרעה בידי וגומר ורביעי ברכת המזון

Why does Rashi gives two different reasons for the 4 kosos?

If you were reading this blog close to 20 years ago you know there is a chakira of the Brisker Rav (stencils Archin 3) as to how the principle of af hein works.  Without af hein, women would be exempt from the mitzvah of 4 kosos because it is a mitzvas aseh she'hazman gerama.  Yesh lachkor: does af hein simply lift the ptur of zman gerama, or is it a new mechayeiv, i.e. even though the mitzvah may be zman gerama, in this case there is a new reason for women to be chayavos?  

Tos and Rashbam disagree as to whether af hein means women were part and parcel of the dangers of galus Mitzrayim, or whether it means they were the cause of the geulah (see this post for a nafka mina):

באותו הנס. פי' רשב''ם שעל ידם נגאלו וכן במגילה ע''י אסתר ובחנוכה ע''י יהודית וקשה דאף משמע שאינן עיקר ועוד דבירושלמי גריס שאף הן היו באותו ספק משמע באותה סכנה דלהשמיד להרוג ולאבד 

It could be that this machlokes hinges on the chakira of the Brisker Rav. According to Rashbam, it's not enough to simply equate women with men, to say that they shared the same pains of galus.  Af hein demands coming up with a special reason to be mechayeiv women because af hein is a mechayeiv in its own right.

This could be why Rashi gives two different reasons for 4 kosos.  Rashi on the Mishna is explaining why even a poor person has to have 4 kosos, and so he invokes the reason on ארבעה לשוני גאולה האמורים בגלות מצרים.  However, that's not a good enough reason to be mechayeiv women. These leshonos that are part of בגלות מצרים, which women experienced alongside the men, would just equate them to men, but af hein demands creating a new mechayeiv. Therefore, Rashi on daf 108 refers to the kosos in Saf haMashkim's dream.  Targun Yonasan explains Yosef's interpretation of that dream as follows (Braishis 40:10):

ואמר ליה יוסף דין סוף פושרנא דחלמא תלתי מצוגייא תלתי אבהת עלמא הינון אברהם יצחק ויעקב דמן בני בניהון עתידין למשתעבדא למצרים בטינא ובליבנא ובכל פולחנהא באנפי ברא ומן בתר כדין מתפרקין על יד תלת רעיין     

The three "shepherds" of Moshe, Aharon, and Miriam delivered Bn"Y from galus; they were instrumental in bringing about the geulah. This sounds like Rashbam's approach of seeing women as the cause of the nes, not just participants in the experience.

(See the Kli Chemdah on our parsha who also discusses this stira in Rashi.)