I looked in Mr. Speer's manual #13, and you're right on the money. If I wuz gonna make one of my famous sorta-scientific wild-eyed guesses, I'd say that since his load is for a jacketed bullet, and you're using cast lead bullets, you're getting the higher velocity. (Softer metal = less resistance in the barrel.) Off the cuff, the difference is a bit startling, but that's about the only reason I can think of.
Reading further in the book, I see a .38 Special listing for 4.7 grains of Unique giving an MV of 815 ft/sec, for a lead bullet. This is shown as a max load in the Special.
I dunno. Try 5.0 and see what happens.
You're from BATFE? Come right in! I use all your fine products!