Doubtless you will receive plenty of replies from members informing you that their payback is "the icing on the cake" and you should be grateful for what you receive, or the other oft-quoted old chestnut is that other networks don't give out payback so in this respect giffgaff is wonderful.
Of course, that isn't the point. Whatever your thoughts on payback - and mine are that it is absurd and over-generous, and to drive up quality input should be scrapped altogether - it is perfectly justified to conclude that since we do have payback it has to be awarded fairly or not at all, and clearly that doesn't happen. It was conceived with the laudible aims of encouraging and rewarding members who post more thoughtfully and participate across all sections of the community. In my opinion it has utterly failed to achieve either aim, driven out good helpers from Help and Support, and instead encouraged yet more greed. And a quick look around shows that the majority of members spend their time on either one or two forums at the most.
Since payback was changed in 2015 just about no one understands the logic of how it works, including me. I'm sure you are familiar with the thread which launched the new 'improved' payback but for the sake of anyone unaware of it here is the link. There appear to be so many variables that it's no wonder members are either delighted or unamused like you with what they have received. And since calculation of payback was automated I think there's even less logic to the calculations.
My advice therefore is threefold: contact Alex (and ask him to do a manual calculation and perhaps explain your payback allocation compared to other months), adopt a "what will be, will be" attitude to whatever you receive, or start posting replies such as "Wicked idea", "Thanks very much for letting us know" or "I agree" and see if it helps for next month's payback. 😉