Manufacture ratings should always be considered as guidelines and never hard fast rules. In my opinion, almost all of these capacities are over rated. The reason for my opinion is that manufactures typically complete their product testing with the gate valve and air valve(s) wide open to calculate the maximum flow rate and maximum air intake using these measurements to calculate the maximum rated capacity for the skimmer. As we all know, the gate valve and air valve(s) will need to be adjusted in order for the skimmer to work properly which will also lower both the flow rate and air intake of the skimmer from the manufacture’s measurements. This is why many hobbyist prefer to use skimmers that have a much higher rated maximum capacity than their aquarium.
The below link can help you determine how to pick out a skimmer for your set-up
And the below will offer you some other ways to evaluate potential skimmers
And some interesting skimmer information