It makes no sense to me at all....you said you got the fees directly from the management company? did they tell you why? The only thing that occurs to me is that maybe a previous owner was paying on a special assessment on a monthly basis and the other units paid it in bulk? Really don't know.
It just doesn't make sense.
The best source for an answer to your question regarding the difference in fee is the HOA itself.
I agree with Marge that the HOA fees are determined by vote via the HOA Board, negotiation is unlikely.