Because if you are at all familiar with Amazon, you should realize that they price things in a really rollercoaster manner and often their prices will return to prior lows.
For example - I've a lot of workshop (~5,000 sq ft) which I needed lighting for. But I couldn't afford to invest in all of it at once. So I'd buy 10 packs of lights at a time.
Here's the graph on their prices.
I bought first at $80/pack, then again at $60/pack, then again at ~ $60/pack
View attachment 747425
So seeing the knee pads are a "commodity" item I wouldn't be surprised if the price fluctuated highly for a while.