@Colin ********
I'm no expert either, but my experience is the opposite of yours. I'm a member of several groups regarding visa issues in Thailand, and although I also have noticed the same as you, that officers interpretation varies, the admins and moderators who has been living in Thailand for decades says that for an extention of stay, the general rule are that a proof of income is more than enough. They do have encountered officers who claim you have to have a bank account too, but then they just went to another office in the region and got their extension there. Bank account are in other words the exception to the rule. But there is one thing many people do wrong, and I made the same mistake; Always bring the original affidavit from the embassy. Immigration will not accept a scan/copy!