This has happened to me and I think this might be your reason why you see a different price. Sometimes BestBuy does offer a different price but the tradeoff is that it is an open box item. When a product has an available open box, it will show you the different prices. Sometimes availability of an open box depends on your location; it looks like they do open-boxes within a certain distance from your location. So, if you are doing a search on BestBuy.com for a particular product and the browser has a different zip code (auto fills the zip code) other than your location it will give you the price for the wrong location. Once you enter your correct zip code at checkout (or before) it will change or state that it is not available. I live in Florida and many times my browser sometimes gives a zip code from different states, like NY or NH.
Another possibility is that an Open-item was available when you started doing your search and just before deciding to checkout it is no longer available because had already bought it online or in the store. Nowadays, many stores are fulfilling their online purchase with their own store inventory, not from a warehouse.
BestBuy has 3 types of Open-Box products:
1) Excellent
2) Satisfactory (in-store only)
3) Fair (in-store only)