Because the wet & contaminated figures already have large margins applied to them compared to the dry...in the non-normal case these margins may not necessarily be appropriate any more.
Try putting some numbers through and you'll find that the LDRs for wet and contaminated rwys in the event of a failure are still greater than that required for a dry rwy. The reason it 'looks' wrong is because airbus keep the same process of landing distance calculation regardless of a/c or rwy condition - the only number that matters is what comes out at the end!