What most likely is happening is that Nitrogen is giving you rigged seeds (which will verify as "fair" later) when you don't change your client seed. These rigged seeds give you 14,15,16 while the dealer gets 20,21.
That is why Nitrogen sucks, they are scammers but with this theres no way to 100% prove its validity except that people lose way more often than they should with Nitrogen's blackjack.
That actually might be a very valid way they might be able to take advantage of it. Actually very likely, considering that such things do not happen on Dice, as the player has many choices unlike on BJ where its the standard play and just had to beat the dealer.
Yes, I have read about that before that site owners changing your seed which would cause your betting style to lose on it. But since most of the dice sites do not change the seeds unless you want them to, it is hard for them to implement it. But if you are someone who is changing their seeds every now and then, then there is definitely a possibility for that. I guess the only way around would be to remember to change your client seed everytime you change the seed.