Simple explanation: both matings occurred, but Neanderthal communities went extinct. So since the child is reared in the community of the mother, we only get Neanderthal male admixture

[–] AbouBenAdhem@lemmy.world 16 points 17 hours ago

That was my first assumption on reading the title, but the article mentions two other things:

The male-gene bias apparently persisted for subsequent generations after the initial human/Neanderthal pairing: male children of mixed ancestry had more offspring than their female siblings
In Neanderthal communities, the bias was reversed (i.e., more human DNA was retained in the X-chromosome female line.)