OP_CSV requires that the transaction consuming this UTXO have an nSequence greater than or equal to the value in OP_CSV.
So, the UTXO containing OP_CSV was created 100 blocks ago, and we set the relative timelock only 20 blocks, but now to use this UTXO we need to set nSequence to 20 blocks, so the timelock ends up being 120 blocks.
Is that correct?
If so, this means that in most cases the relative time lock will be greater than the value set, unless the UTXO is used in the next block immediately after it is created.