Possibly the reason the CM Imm is asking for 'proof' of income lies in the fact that the US Embassy doesn't ask for 'proof' when they notorize the income letter.
Any American can get the notorized letter simply by swearing on the bible that their income statement is true. Lots of lies being bandied about going on there I'm certain.
An American I know here in Hua Hin lies through in his teeth everytime he goes his Embassy looking for the income letter. He laughs at it.

Many countries' Embassies, when providing an income letter, demand physical proof that the amount provided is true. For instance, the Canadian Embassy asks you to provide your latest income tax filing. Only after seeing that will they notorize the income letter.
In 10 years my income letter has never been questioned by Thai immigration.