I would be VERY careful about reading anything into those charts...

I would be VERY careful about reading anything into those charts without a lot more justification. They look to me to be most likely a raw count of historical reports. The count is dominated by low-intensity events, which prior to the development of the seismograph network would have been effectively invisible to anyone outside of a few dozen kilometres from the volcano. Even in the early 20th century, vast areas of the Earth's surface would have gone completely unrecorded.