Hi Alec and Dan,
The technical reason for this is that the 1-month and 1-year averages are not using pseudo averages, and only pseudo-averaged data is available for map markers. For reference, you can view pseudo averages in our API docs (this link will only take you straight there in Chrome). You can also read about the difference between map marker and graph data on our map.
We’d like to synchronize these values in the future, so that the map markers can display true averages instead of the pseudo averages they currently display. However, we don’t have plans to do so just yet.