Mostly they do not. Of the 368 prices we have checked more than once, 340 have not moved at all and 28 have. That rests on 10,903 readings taken over 52 days, the first on Aug 25. It is a short record and it is the whole record: nothing here is an average of anything, and nothing is a forecast.
Of those 28, 13 moved by more than the 10% this site treats as notable, and the smallest move we recorded is 1.7%. The widest is BulkSupplements.com Whey Protein Isolate 90% (1 kg), between $54.97 and $78.97 in that time. We do not know why any price moved and this page does not guess.
What this is useful for: knowing whether it is worth waiting. What it is not useful for: predicting. A price that has sat still for a month can move tomorrow, and every product page shows its own history so you can see the shape of it rather than take a summary's word.
One thing to hold in mind about the table: a move shows up here only where we can re-check a price often enough to catch it. Retailers whose prices we can read every day are over-represented in it, and that is a fact about how often we look rather than about how often anyone changes a price.
Every figure is a price we recorded at the retailer named on that product's page, on the day we recorded it, and the lowest and highest are the lowest and highest we saw rather than the lowest and highest there were. Prices we cannot re-check are left out of this page entirely: a last known price is not a record of anything moving.
What this rests on: prices we last recorded at the retailer named on each product page, which go stale, and each page says when it was checked; the products we track, not every product sold, and Amazon and Walmart are not among the stores; listings and labels read at the source, each linked from the page. Found an error? Tell us. The full method: How we compare.