They were calculating for two sandwiches. A slice of cheese at in-and-out (not going to use Kraft slices because idk what better-than-mcdonald's fast food place would use such a cheap cheese) is 110 calories, several slices of bacon per sandwich is probably 100-150 calories...together that's anywhere between 210-250 calories, times two (two sandwiches) and that's 420-500 calories, which coincides with what OP estimated. In-and-out uses damn tasty cheese though. Never been to Burger King but I'm sure their sandwich menu has calories notated somewhere.