That is obviously much more likley to be the case with pre-17th ed. instals, notably 'split load' ones, but a lot of installations under current regs will have all circuits having ('additional') RCD/RCBO protection, and therefore technically do not require an additional 'up front' device (if they are TT) to provide earth fault protection.
As I think I said before, my installation is like that. All of the circuits in my (TT) installation have RCD or RCBO protection ('additional', if you must) and therefore I don't actually need the 'up front' 100mA Type S RCDs - but I've kept them as isolators and as belt-and-braces ('additional additional', if you wish).