It depends. What the store sells refers to the weight of a pair, and it is the standard weight, not the actual weight; Generally speaking, the weight of dumbbells in the gym refers to the weight of a single dumbbell.

You can look at the weight of each dumbbell piece, and then weigh the weight of the dumbbell rod. These add up to the weight of a dumbbell. Usually the dumbbell rod is about 2 kg. Generally, the dumbbell quoted in the store will be heavier than the actual weight (it is not clear whether this weight is a dumbbell rod).