In fact, the difficulty tuning was usually harder within the first release of the game(regardless of location). So games that saw a US release first were hard there, while games released in Japan first were hard in that version. And since the majority of the NES library derives from Japan-first releases you get the "Japan version is harder" effect.

There are some exceptions to the rule of course, but it usually holds that the later localizations get the more polished product.