There is no way to tell, at this point, if it's the builder or the management company cookin the books, if that is indeed what they are doing. The difference in billing rates could be the difference between a calendar and fiscal year end, in other words, an accounting methodology, not fraud. Or it might be the difference between budgeted and actual amounts, which should then leave an excess in reserve.
There is a mutual dependency between the developer and current owners. It's common for potential buyers to talk to existing homeowners about the HOA and it's in the developers best interests to maintain good relations.
The last thing you want is for the developer to fold and not finish building and selling out the sub division.
Someone has to be Columbo and do the " there is just one more thing that's bothering me" routine. Good luck.