Tempura vs. Tenpura

In Japan, a very commonly made food is tenpura. Tenpura is basically battered and fried vegetables and sometimes shrimp. This is usually a main dish in Japan, but can be a side dish to bigger meals. In America, it's called tempura in many Japanese restaurants. Americans love anything fried, and it's often served as a side dish to sushi. In a local restaurant in my area, it's one of the most popular side dishes to sushi. This restaurant also sells Chinese food, and many of the dishes are fried as well. These dishes are considered very delicious. All of the people who eat at sushi restaurants in America, especially sushi enthusiasts, usually enjoy sushi. I mean, fried butter is a food that's commonly served at many state fairs in America. Of course tempura is popular. I'm not sure why, during the transition from Japan to America, the n in tenpura changed into an m. This confuses me because in my opinion it's easier to say tenpura than tempura. In Japanese, the hiragana for tenpura is てんぷら. The "ten" in tenpura is the beginning kanji of the word "tenki" or weather. This is what tenpura looks like with its first kanji included: 天ぷら