Was Satoshi's target recalculation off by one?

So, I was checking out Karpathy's take on building Bitcoin from the ground up in Python, and he made a point: He seems to be spot on. Shouldn't that number be 2015 instead of 2016? Or am I missing something here?

It was probably actually an off by one bug, OTOH, dividing by one off there is the minimum variance unbiased estimator for that process (it's scaled by (n-1)/n), so in that sense it's the correct measure (perhaps accidentally).

I think it's not off by one bug. It seems, that the genesis block took several days to mine - mined with much higher difficulty - so it was omitted from the difficulty calculation.

Gotta love that we're still reverse-engineering decade-old code trying to figure out if the creator was playing 4D chess or just shipping fast and breaking things before that was a Silicon Valley mantra. Karpathy's Python implementation is solid homework if anyone wants to actually trace through this themselves instead of armchair theorizing.