Not sure. Could it be the chips surrounding the NOAC on the PC board, don't know. I do know that everyone does have their particular twist when it comes to the NOAC that they use in their clone consoles. Some characteristics are inherited as it seems that everyone has built their take on the chip from the same root source tech. Hyperkin has been very successful with the Genesis on a chip being that it is playing Virtua Racing, a first as far as I know. It is evident that they are working on the NOAC to make it better. It seems that the NES is a very hard system to backward engineer. One of the characteristics of the NOAC is that there are 23 games in the NES library that it will not play. Been that way for any clone that was out there. Only recently are you starting to see compatibility with Castlevania 3. There are some clones that are better with the color pallet, Gamerz Tek clone NES has a better color pallet than the Hyperkin unit here. There have been a lot of improvements over the years. I still have my Generation NEX which it has trouble and issues with certain games, graphical glitchs for example.
Most hardcore purests hate the NOAC "as it is" due to the flaws in game playback, or compatibility. I find the gameplay intact even though there are the sound effects that may be off in pitch. And the grass may be brown in some games instead of green, like Jurassic Park. Instruments, notes, in some of the in-game music is very different than what they should play. I personally have an advantage, or it may be a dis-advantage, all according to way you look at it. I have never owned the original NES. So I do not really know how the games should look and sound. Even though there are those issues I'm having fun and that is what matters to me. People out there, mostly casual gamers, are purchasing these things and having fun. I sometimes look at youtube just to see how far off a game it. Gives me an idea of the unit's compatibility.
edit: Oh no!! The money bags in Castlevania have a pink tint! We're all gonna die!!