The FBO manager at KMZZ said that he often gets a call from various fuel pricing resources (GobalAir, ForeFlight, etc.) inquiring about current fuel prices. He also said that he does not remember ever getting a call from iFlightPlanner regarding fuel prices.
Since their website showed the same outdated price as iFlightPlanner, I asked whether their website auto-updates from another resource. Turns out, he manually updates their website but had forgotten to do it this time, so he updated it then. Shortly after, iFly and iFlightPlanner both listed the correct fuel price (updated about 3-5 days ago). So, it looks like iFlightPlanner is getting the pricing from the airport website.
There are certainly a lot of variables here. But at least there seems to be a reasonable explanation of what happened in this case.