The Japanese transliteration of "computer" (コンピュータ) is written with the "n" (ン) character, but it's pronounced more like an "m" sound because the lip movement for the following sound modifies it. The same dichotomy is seen with shinbun/shimbun, for example. Of course, "Famicom" is the official way to romanize it, so it's usually preferred.

Same thing happens in English: incredible vs important. input is still spelled with an n but pronounced conversationally, you'll get imput.

It depends on the accent; I certainly say "input" and "incredible" with an n sound.

I pronounce incredible with an n sound as well. I included it to contrast with important, showing how the following consonant influences its pronunciation. When enunciating, I pronounce input with an n; when conversationally, with an m. Different dialects are of course going to be a factor here as well.