Parsing his sentence for tiny nuances like that isn't very helpful IMHO, but I'll indulge you.

His exact words are "these differences may explain". He doesn't say "these differences may PARTLY explain". If I say that A may explain B, the reasonable implication is that A may fully explain B. So, yes, he does suggest that non-bias causes are the only causes.

Just to be clear: I don't think this makes any real difference. The reaction to his email would've been the same either way. But the fact remains that your interpretation of the quote isn't supported by the actual words he used.