Most bonded bullets of straight cup and core design, use pure lead for the core. This is what I read about them (in general) - I have no experience with them, myself. They are very rapid expanding - wad up like bubblegum and thus oft times do not give the deep penetration their weight retention would suggest.
The Swift A-Frame design, having the partition resists this gross expansion and gives back penetration that would have been lost, had it been a standard cup and core design.

This made sense to me. 9.3x57's penetration tests of 300gr. .375 Interbond also showed this "gross" or large expansion and reduced penetration of the standard cup and core bonded bullets, compared to the Swift A'Frame.