The parameters of charum

In Emor we read that a charum cannot function as a kohen.

What is this blemish? Rashi writes:

with a sunken nose: Heb. חָרֻם. [This term means] that his nose is sunken between his two eyes, such that he applies [eye shadow to] his two eyes with one stroke [i.e., his nose is so sunken that its bridge does not intercede between the two eyes]. — [Torath Kohanim 21:48; Bech. 43b]

חרם: שחוטמו שקוע בין שתי העינים, שכוחל שתי עיניו כאחת:

Ramban notes that this is actually a machlokes in both the gemara in Bechoros and in the Sifra, and the Chachamim there say the nose need not be recessed that much:

יח): חרם או שרוע - 

כל שחוטמו שקוע בין שתי העיניים שכחל שתי עיניו כאחת, לשון רש"י. 

ובתורת כוהנים (פרשה ג ז) ובגמרא דבכורות (מג ב): שנינו: 

חרום שחוטמו שקוע. 
חוטמו בלום חוטמו סולד חוטמו נוטף מניין? 
תלמוד לומר: או חרום. 
אבא יוסי אומר: אין חרום אלא הכחל שתי עיניו כאחת, 
אמרו לו הפלגת, אע"פ שאינו יכול לכחול שתי עיניו כאחת. 

ולשון "חרום", מלשון כל חרם אשר יחרם (להלן כז כט), והחרמתי את עריהם (במדבר כא ב), עניין חרבן, ויקרא חרום, כי החוטם הדרת פנים, כמו שאמרו (יבמות קכ א): אין מעידין אלא על פרצוף פנים עם החוטם, ואשר אין חוטמו כתאר בני האדם צורתו חרבה.

Rabbi Eliyahu Mizrachi is troubled by this selection of the yachid position over the rabbim. He writes that he doesn’t know what compels Rashi to select the yachid position:

Gur Aryeh tries to answer this:

He points out that the machlokes mentioned in the gemara is in a brayta, but that the stam Mishna is like this solitary position. And see the supercommentary on Gur Aryeh there where they quote Nachlas Yaakov who quotes Yevamos 42b that if the Mishna is stam and there is machlokes in a braysa, the halacha is like the stam Mishna.

He also gives another, less convincing answer that in this situation, the fellow is certain considered charum.

I agree that it certainly makes for a more striking and memorable definition of this particular blemish, whereas just the word shakua (recessed) is not something that a person would readily understand. The painting of both eyes at once gives a very good visual.

Is it possible that Rashi just went for the simple explanation as found in the Mishna? Perhaps. But see how he is continuously referencing Toras Kohanim (Sifra) throughout his running commentary.