Why is the relative abundance of helium substantially higher than the abundance of hydrogen in stars of the Wolf-Rayet type

Why is the relative abundance of helium substantially higher than the abundance of hydrogen in stars of the Wolf-Rayet type, in contrast to the overwhelming majority of stars?

Wolf-Rayet stars are the bare cores of massive (from 30 to 100 solar masses) and hot (from 30 to 40 thousand K) stars of spectral classes O and B, which have lost a significant part (up to 60%) of their mass due to the outflow of matter.

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