Check the oal the data for the 357 may be for flush seated and the 38 for a nose out . Also different brands may have deeper HB cavities or different shapes .

See a Speer manual article "why ballisticians get grey" . The parameters for HB WC are to have every bullet exit the muzzle about 900 fps and not blow out the skirt not to avoid a particular pressure limit .

The data may reflect a particularly tight 357 test gun or a loose 38 . It may be 60 yo 38 data simply pulled forward and data from a pressure tested 357 in a solid 4" universal receiver .
Lot variations in the powder , especially if it was old data just pulled forward vs new tested .
I don't remember where but someplace I swear I saw a whole article dedicated to Wadcutter Brass and how it was in some makers more than just special canlure brass having more interior case taper below the groove to reduce case volume for competition shooters and so on .

There's 100 reasons for the difference but the biggie is that you want low muzzle pressure and about a 900 fps MV for them .