exchange by charge – Why does RBF rule #3 exist?

The mempool-replacements.md doc states the next guidelines for RBF:

3. The alternative transaction pays an absolute charge of a minimum of the sum paid by the unique transactions.

4. The extra charges (distinction between absolute charge paid by the alternative transaction and the sum paid by the unique transactions) pays for the alternative transaction’s bandwidth at or above the speed set by the node’s incremental relay feerate. For instance, if the incremental relay feerate is 1 satoshi/vB and the alternative transaction is 500 digital bytes complete, then the alternative pays a charge a minimum of 500 satoshis greater than the sum of the unique transactions.

Is not rule #4 strictly stronger than rule #3? I’ve all the time seen each acknowledged as unbiased guidelines (even in code implementations), and this makes me marvel if I am lacking some subtlety right here.