psp memory card went from a 16G to a 15G!?

psp memory card went from a 16G to a 15G!?

I bought a 16G magic gate memory stick for my psp and i downloaded a whole bunch of games on it and had 47 MB left and then i went on usb mode and deleted some of the games on my psp and it still have 47 MB! why? and when i deleted every game on my memory stick it says only 15G when it’s suppose to be a 16G! anyone know why?