The algorithms do have a problem if you have a debit balance, as they seem add that to the estimated annual usage figure to get you pay it off (or something like that).
Although I just tried that with your bill and it didn't work. (39.20 x 12) + 190.52) / 12 = £55.08. So who knows.....
Some companies have really crap accounting systems and customer service is hugely variable. Best advice is to keep a reasonably frequent check on what you use, what you pay, and your balance, bearing in mind your overall annual usage.
So Energy here. Currently about £390 in credit and paying £110 a month direct debit, reduced from original £175 due to government rebate.
Credit going down all the time because of winter usage and expect to be in a big debit by end of winter, so might need to be paying more - don't really know...