The maps were developed on a dyno with a bike, just not your bike.
A dyno doesn't produce anything other than figures. The developer runs a bike up on it then uses the data from the dyno to inform changes to the code in the ECU, then repeat the process until they have something that is optimised for that specific setup. If you have anything different on your bike from what was on the test bike, then it won't be as optimal.
Generic maps can be better than stock, but not as good as an individual setup, but they are cheaper as the cost of the setup is shared across the expected sales of that map, whereas you are paying all the cost yourself for a custom one.