It is likely the tanning process which helps slightly. In any case ABC (Kodak D-1) doesn't produce such heavy stain unless you alter the dilution for less sulfite. Kodak later came up with formulas which were designed for more imagewise staining.

I'll also throw in my white elephant for the fun of it: why anyone would bother in this day and age with DBI versus time/temperature control is beyond me. In the days of Weston it made some more sense because light meters were primitive, and materials were subject to more variability (film speeds, developer activity, etc.). At this point, what value can it possibly add?