I think it has been common practice for Imm to ask for 12 month bank statement.
I remember 2 years ago, when I went renewing my non-o at Nonthaburi. The bank letter and a book updated was not sufficient. They asked for a 12 month bank statement signed and stamped by the bank. Once submitted, they accepted my documents but after 2 weeks, immigration called to provide the current month statement, once again signed and stamped by the bank. I did provide it and everything went fine.
My wife asked them why is this require because it does not even feature in the requirement list, the lady answered that because there's no proof of income. The top gentlemen wanted to see if I have income or cash flow before approving my extension as my account was always idle. The year after, I did prepare all related statement etc., there was no issue.
However, last year, I started work again and was able to provide a work permit together with a 12 month bank statement, it was flatly refused. We were told that the work permit was enough together with the bank letter and a bank book update.
I hope that my little experience help others in that matter.