Why Renown's thinner armour was not the whole story

Why did the Renown class carry less armour than the earlier Lion-class battlecruisers?

The ships used less armour but placed it more carefully, gaining some advantages from distribution even with a lighter total scheme.

Total armour weight is not a complete measure of protection. Renown carried less armour than the earlier Cats, but it was distributed more carefully. That choice did not remove the design's vulnerability. It does show why location matters alongside quantity when comparing battlecruiser protection schemes.