Calculating free parking spaces is not easy

Today, I drove my son and my husband to their gym (my husband does the training part but the gym has a room for kids) and when I went back, it was pouring rain. It being the salary weekend I guessed the parking space would be cramped so I was happy to see the sign saying it was 355 places free. I've always wondered how that works, the calculation of free spaces. I now found out: it doesn't. I spent the next 15 minutes circulating the parking area with an increasing number of cars doing the very same thing. Finally, I gave up and just parked very illegally and waited for my husband to come out. (Since my parking would have hindered 5 cars from pulling out, I had to stay put.) So, now I have to do some reading if that remaining parking spaces is just a scam or if it really works, somewhere. It doesn't work in Farsta centrum, that's for sure.