As Rashi explains, Tzafnat means "hidden things," and Panei'ach means "he reveals" - i.e., Joseph was able to explain things that were hidden. Why, then, didn't Pharaoh call Joseph Panei'ach Tzafnat, which would have been more logical? To teach us that the real reason Joseph was able to interpret dreams was as a reward for concealing his righteousness. Because Joseph conducted himself in a humble and unassuming manner, "Tzafnat," he merited the gift of "Panei'ach." (Sefat Emet)
Reprinted from the Parshat Mikeitz 5761/2000 edition of L’Chaim.