It's more that servi simply weren't included in the returns from the sixth circuit (covering the East Midlands and Yorkshire) - each circuit was independent, and decided for itself what to count and how. That led to some surprising inconsistencies, like this one.

The site links to a decent explanation - https://www.domesdaybook.net/domesday-book/structure-of-dome...

Also a good write-up about servi (traditionally translated as 'serfs' rather than slaves, though they meet the modern UK legal definition of slavery) - https://www.domesdaybook.net/domesday-book/data-terminology/...