Immigration rules misconception

A list of required documents for visas and extensions can be found at Thailand Immigration Bureau
Download forms here immigration.go.th...download
One must understand that these are the basic requirements.
Each jurisdiction may require additional documentation at their discretion.

There seems to be an omission in the basic requirements where proof of income is done with an embassy or consulate certified letter.

Screen copy from www.immigration.go.th

A bank letter is also required along with your original and a copy of your Thai bank passbook.

Let's say you're in the middle of your monthly spending cycle and you only have a balance of Bt30,000 when your visa renewal is due.  Immigration officials can verify the money flow from your certified bank records and grant you the extension.