I got a Thai tourist visa in Australia. When I arrived on 3 August the Immigration officer stamped valid until October 31. My understanding was that the visa was for 60 days. This would mean the end of September. Why the difference?

Their mistake, still treat it like a 60 day though

 

Thanks. But someone said I can apply for an extra month on this visa by paying 1900 and filling in a form at Immigration. Can this be done at Soi 5 Jomtien?

Yes.

The immigration officer picked up the wrong stamp. You can get it corrected at immigration.

You can get a 30 day extension of your entry. You could go to immigration in September and apply for the extension and get the stamping error fixed at the same time,