In describing the layout of B'nei Yisra'el's camp in the wilderness, the Torah records four groups (called d'galim) comprising 3 tribes each, and lists them in order, dictating a diagram of how they traveled. After finishing with the lead tribe in each group the Torah introduces the next tribe using the formula
וְהַחוֹנִם עָלָיו מַטֵּה X
And camping adjacent to [tribe Y] was tribe X
The preposition that the Torah uses for "adjacent" is "עָלָיו", which could be misinterpreted as its more standard meaning, "upon". Thus, Unkelus translates the phrase unambiguously as
ודי שרן סמיכין עלוהי שבטא דX
And those who camped adjacent to it were tribe X
This is the translation three out of four times this formula appears (in Num. 2:5,12, and 27). The fourth time, when a slightly different phrasing is used (in 2:20), leaving out the verb, Unkelus follows suit, literally translating
ודסמיכין עלוהי שבטא דX
And those adjacent to it were tribe X
Only this time does Rash"i comment, seemingly to explain the meaning of "עליו", by quoting Unkelus. His comment reads in full:
ועליו - כתרגומו ודסמיכין עלוהי
"And upon it" - Like its translation [by Unkelus]: "and those adjacent to it"
Rash"i is assuming we were able to figure out the meaning of this word the other three times either on our own or with the help of Unkelus, but this time not. Why is this? What is he uniquely adding to our understanding of this pasuk?