Because your client's tax is computed based on the tax table instead of tax rate schedule and the net change in taxable income is within $50?

No one can tell except for you as there could be more moving parts than just the expenses.  Follow the $$$ and you should find the reason.

Whenever there's a change in certain numbers, I find it helpful to do a back of the envelop computation to rationalize the change I expect and then match my estimate up against what the system spits out.