It says that males just can't reach the heights of flexibility and athleticism that the females can.
Presumably a lot of them either stay as a strong Wych fighter, but not getting much further, or become reavers/pilots... I guess the Dark Eldar share many human's opinions on woman drivers
"Oh how awful, did he at least die painlessly? To shreds you say? Well, how's his Dracon holding up? To shreds you say? Very well then... Sad, sad, terrible gruesome news about my colleague Archon Mhu'bhutu." - 'The Feather', Dracon of the Bladed Lotus