Fuel gauge problems why only show half tank when full?

They can replace the fuel pump and not the sending unit and it will do that. The sending unit is the float inside of the tank that tells the cluster how full the tank is. That is why when I do these things, I replace the fuel pump and the cluster items including the float all on one shot.

I had this ford escape 2009 model, had change the fuel pump assembly with a new part, and also the fuel pump module ath the same time. Then i had the tank full and its been okay for the time until i had to refill again. Afterwhich i got a almost empty tank with the light coming on, then at the petrol Station i had almost 25 liters and saying its already full tank. I tried 2x succeeding refill at different times with comparison to my mileage everytime i make a fuel refill that tank is not reaalt filling up but instead with air pockets as commented by the petrol station guys. Any ideas to fix?