Tourists flock to El Salvador despite Bitcoin bear market

I’m reading the yellow paper and having some problems understanding equation 169 from here – also below:

s(U,H) = (P(H) = P(U) ∧ H != U ∧ U !∈ B(H)_U) 

I can understand the first two conditions:

1. The parent of the block associated with header H must correspond to the parent of the block corresponding to header U. In other words, they must have the same parent.
2. The blocks must be distinct

However, I cannot understand why 3 is needed. If the parent of U must correspond to the parent of H (i.e. they have the same parent) then how can U be a part of the ommer list of block H? Blocks can only be part of the ommer list of another block (as far as I understand it) if (at the very least) they are siblings with the parent of said block. So, given our initial condition that the parent of U must correspond to the parent of H, why do we need to additionally state that U cannot be part of the ommer list of H.