Macbook Pro 256gb (2018 model) only has 175gb available.

So I bought a second hand MacBook Pro with a 256gb SSD but it shows I can only use 175gb. When I go into 'manage storage' it says that the system only uses 20.85gb. What would the rest be used for? I don't see any partitions with disk utility. Is this normal? I've read online the usable capacity for 'user files' should be around 220gb - 240gb

I’ve completely erased the SSD and am busy with a fresh install now. But after I did the erase it still said only 175gb available.