"Ach es Mateh Levi lo sifkod - Don't count Shevet Levi" (1:49)
Rashi says that Hashem foresaw that there will be a decree on all those who were counted from age 20 that they will die in the desert, so He said, "These shall not be included, because they are mine, for they did not stumble with the Golden Calf."
The Sifsei Chachamim points out that Shevet Levi did sin with the Spies along with the rest of the Jews.
There is a major difference between a sin that comes after another sin, and a sin that comes after a victory over the Yetzer Horah!
We may think at times, "What use is there to my refraining this once? I doubt that I will be able to maintain this lofty level forever. I may as well succumb right now." This is faulty thinking. One moment of stalling the Yetzer Horah's advances is immensely special in Hashem's eyes! In that short instant we become "His", and even if we later fall, we have already accomplished so much!
Even though the Levi'im did not withstand the test of the Spies, they did not deserve the same fate as the rest of the nation. The Levi'im had an earlier victory on their account!