That could just be caching and interruptions, but I wouldn't really consider more speed to be a problem.
That can sometimes just be bad behavior in the tool tracking the download speed. Interruptions will throw a brief period of zeros into the calculator, and then when it realizes it has sent more than that it revises the speed temporarily. If you are seeing flat consistent increases in speed over long periods, your ISP may just be upgrading the circuit. Sometimes it is cheaper (or just less effort) to upgrade equipment and signal strength for everyone in a given area.
Just noticed the other day that it looks like they doubled my internet speed, I don't believe I was notified. Probably need to consider dropping it down a speed level.