It's because the slot it gives you can only get the relevant vehicle you have to pay with credits for a slot that any unit that can use and its the same with ATF and DEA expansions
Ok, let me put it this way. It appears you bought the Water Police Expansion. When you do that then you get two spots BUT they are only for Water patrol boats. Your current station level is 0 so you only get 1 vehicle. You have to expand that in order to get other police vehicles and game warden trucks.
Thank you, especially Kaziya, I wish there was some documentation or warning that this happens haha. Thanks for the help.