It probably means that your average varies randomly and that this particular day had it start high and end low. Your typing speed varies from test to test and that variation doesn't mean much, as you've probably realized. The average that you see on the site only shows the average of the last ten, so while it varies less, it does vary. If it showed, e.g., the last 100, it would be very hard to change, and would generally mean that there was some kind of long term (hopefully upwards) change going on.
Another possibility is that after a while you got tired, so while you were genuinely typing worse, it's not permanent. I sometimes find that after a while typing, I make a lot of mistakes. It feels a bit like saying a word too many times and having it stop sounding like a real word: the different positions don't really seem that different.
Anyway, I wouldn't worry. It's the trend over weeks, not hours, that matters.
I will add that if you do make it to 200 WPM, you'll be one of the fastest English language typists in the world, and that 100+ is good. Guinness lists Barbara Blackburn as the fastest (recorded doing 170 WPM for short periods, and her peak was 212 WPM) but there is some debate on how to measure that. This Quora post
https://www.quora.c… seems to have a good debate and information.