What is the publish date of the one vs. the other?
Both are the same date. the 21 Gr min is from the Hodgdon data, the 19.9 maximum from the Hornady data of the same year.
Case volume & seating depth:
Good thought I dug out both manuals to see what those were for the 2 sources.
Hornady says 1.590 (these were fired in a Colt Python, with an 8"Bbl length.) & doesn't give pressure directly, JUST m/v. they list a range of 17.4Gr min for 1250FPS thru 1500FPS with a MAX OF 19.9 gr.
Hodgdon gives the same COAL of 1.590, but they are using a 10" barrel, so maybe the longer time in barrel & different pressure resulting from that is the major player? They list both M/V & Pressure at 1881 FPS for 38,400CUP min with 21.0 Gr to 1966FPS with 41,400CUP max using 22.0 Gr of powder.