My girl was looking for a dress for Halloween. Yesterday she found one on Amazon for € 35 and put it in the cart, but did not buy it. Today she looked it up again and it was € 50 so she asked me to look it up with my phone with my Amazon account - it turned out to be € 23 for me, less than half of what it’s for her!
Automatically applied coupons, and deals that don’t even require prime could potentially do this without setting the "base price. Unnsire if thats the case, but it would likely not be picked up third party price trackers.