it may be technical problem because google cant cheat anyone,it is very fair and you should contact them too by emailing them to clear the doubts.
same thing happened to me yesterday i was @$180 but today last payment is $112 i think they are no add payment carried over from the previous months.
It keep on fluctuating but dont worry .i would suggest the who ever gets the problem should mail them and tell them the situation.There must be a system problem,any technical one