I had to look it up since I have never owned a Walker but to my surprise, you are correct. I wonder why they call it a .44?
Actually it said they take round balls up to .457 which is really 46 caliber. I do know .44 magnums take .429 diameter bullets as I have owned a bunch and used to reload for them too.
Because that's what folks were used to calling them back in the late 1800s. All the Remington and Colt's percussion .44 revolvers were actually .45 cal: .451 and .454, Remington did make a cartridge revolver in 1869 that used a conversion cylinder on a percussion revolver in .457, a .46 cal. The 1873 44-40 Winchester is a true .44 cal at .429.
Many variables in the Arts of Black Powder,especially long Guns.