The BSA specifies 3 positions for Den Leaders and 2 for Assistant Den Leaders. They are:
Den Leader (Lion Dens, no assistant because parents are supposed to be present)
Tiger Den Leader (Tiger dens, no assistant because parents are supposed to be present)
Den Leader & Assistant Den Leader (Wolf and Bear dens)
Webelos Leader & Assistant Webelos Leader (Webelos dens)
The original Lion Pilot did not have Den Leaders. I believe the BSA just decided to reuse the position instead of creating a new one.
Because Tiger and Webelos leaders are registered with a different position code, they appear as different positions in Scoutbook. If the BSA eliminates the Tiger and Webelos leader positions, Scoutbook will be updated.