This could be sample aliasing. For instance, on my WeatherElement pages, we collect data and save it from most stations every minute, or 1440 data points a day. But our graphs are only 350 pixels wide, and about 260 of that is the actual graph. The graphing routine actually counts how many data points there are in the previous 24 hours (to allow for different upload times), divides that by 320 and uses that number to select the graph points. So, for a perfect 1440 sample day, 1440/260 = 5.5 pixels. So it selects every 5th or 6th data point to graph.

The graphs are cached for 10 minutes, so after that, you are probably getting a slightly different data set graphed.

Which doesn't explain the strange peaks themselves, but may explain why sometimes you see one, sometimes 2.