I noticed that there are some general patterns for the number of sites that a plugin is installed on. In most months, the number of sites installed increases for most plugins. In some months however, there is a drop of a few percent for a lot of plugins. This happened in February 2017, April 2016 and December 2014 for example.
The mostly likely explanation for this is that the total number of sites successfully polled dropped in those months, which could be for any number of reasons.
It would be nice to know the total site count each month, to get a better idea both of the sample size, and perhaps an idea of relative penetration rather than absolute numbers of particular plugins. The https://moodle.net/sites lists current Moodle sites, but this seems from the documentation to be updated weekly rather than monthly, so isn't the exact same sample set as the plugin statistics. It also doesn't show historical data any more (I think it used to).