These errors vary from month to month, and sometimes affect electricity as well as gas. I've been exchanging emails with OVO about this issue for around 2½ years(!), getting responses from a different agent every time, ranging from not understanding the question to understanding it but blaming IT problems. The latest response from Sally Golder on 6 April 2017 said "The issue affecting the 'compared to the same period last year' field in the yearly comparison on your statement is an ongoing issue that is currently being investigated by our technology department. However I can confirm the 'for this bill period you have used' field is accurate, as are your charges.".
Clearly there is a serious bug in OVO's software affecting what should be a very simple calculation, and I fail to understand why this still hasn't been corrected after such a long time. If it really does affect only the yearly comparison then it is not material to what I'm paying, just a massive annoyance, but it's getting to the point where I'm losing confidence in OVO's ability to calculate anything correctly. If this is not sorted out before I next need to renew my contract in December, I shall certainly look elsewhere.
Best answer by RichardP
For the benefit of anyone else interested in this topic, I'm copying in part of the reply I received by private message from Lucy_OVO: "The response I got from our Tech guys were that they’re aware of this but they haven’t got a fix for it yet. It will only be affecting this part of the statement and all reflections of your usage for the month (for the period that you’re being billed), your unit rates and your standing charge - all this information will be correct".
This is rather disappointing, particularly given the length of time since I first raised it with OVO. I'm inclined to conclude that the 'Tech guys' have no intention of fixing this serious bug in what should be a simple calculation.