Another thing to consider is changes in the people you text. If they switch to iPhones, that adds more and more people who you wouldn't see text details for. And if any iPhone users switch to Android or other phones, you would then see them.
So as people upgrade phones, switch phones, etc., it's possible that your usage will fluctuate. I had 5 family members that all had Android phones for 2 years and when it was time to upgrade, they all switched to iPhones. That immediately changed my text usage as now it's all iMessage.