It's pretty easy to get that "Best Seller" badge on a product because a product can be the "Best Seller" for a super niche category while being much farther down on the list in categories that people actually look for. Product categories aren't exactly regulated either. For instance, this meatloaf pan is ranked #1 in Barbeque Turners and #858 in Lawn, Patio, and Garden and it doesn't even have a rank in Bakeware or Kitchen and Dining.

Best Seller rankings are also reset every hour and have a very heavy recency bias, so if you're in a niche enough category, even one or two sales in an hour can put you at Best Seller status