Lead-cored bullets tend to "obturate" (expand slightly) when booted in the rear by powder gas. The rear ends of flat-based bullets obturate quicker, due to more area exposed to the hot gas than the smaller flat area at the end of boattails.
Boattails tend to funnel some powder gas into the rifling, due to the angle on their sides, while flat-based bullets tend to have sealed the bore by the time their rear end passes the start of the rifling.
Or at least those are the conclusions of the research I found.
