Or HaChaim on Numbers 6:9/אור החיים על במדבר ו׳:ט׳

1 וטמא ראש נורו, and he defile his consecrated head, etc. Both the letter ו in the word וטמא as well as the word ראש appear superfluous in our verse. The meaning may be that even if the ritual impurity contracted by the Nazirite was unintentional, not only does his body become ritually impure but also the hair on his head, as the Torah refers to his hair as "the crown of G'd which is on his head."

2Another detail which is revealed by this wording is that not only does the Nazirite have to make up the days following his becoming ritually impure, but even the days which he counted in purity are not accounted for him and he has to go back to square one. This is confirmed by the words והזיר לה׳ את ימי נזרו in verse 12 where the Torah specifically states that the days he observed prior to becoming defiled become void. The reason the Torah says once more כי טמא נזרו, that he defiled his Naziriteship, is intended to prevent us from making the error of thinking that he loses only the day on which he became ritually impure, i.e. the beginning of Naziriteship. The words והימים הראשונים reveal that when such a person has contracted impurity lasting not only one day but a minimum of two days he loses not only these two days but all the days he had already counted. It is what is known as גזרת הכתוב, i.e. the Torah did not bother to explain its reason for this ruling.