I think I figured out WHY it's happening... but I'm not a developer, so not really in a position to fix it.
The issue is likely related to the change between regular time and summer time. The weird thing is that everything is correct during summer time, and off during regular time.
Here in the US, summer time ends in November (an extra hour in the month, which causes two November entries to appear, creating a one month offset in how the data is displayed) and starts in March (one less hour in the month, which causes March to not appear, removing the offset).
I'm not sure if the package is using time based on GMT or local time... or maybe the pieces doing the logging are using one time and the web interface is using another, causing the issue... if someone could look at it, that would be great. This was a package originally created by a member of the pfSense team.
Edit to add: Created a bug report, #9537: https://redmine.pfsense.org/issues/9537