I guess this article explains it
Three months ago on Amazon, a pair of two-toned Wolverine oxfords sold for $160. Last month, I snagged them for a mere $12.54, even though they still retailed for $70-plus on competing discount sites. Had I waited until last weekend to buy them, I could have saved another $2. But if I had waited any longer, they would have gone back up, to $28.06. In suede, my new shoes' price fluctuations were even more erratic, wavering up and down by more than 20% every few days.
You may not not realize it, but shopping on Amazon is more like playing the stock market than shopping at your local big box store. Prices don’t just change with the season or when something goes on sale. They fluctuate as often as every 15 minutes.