How is it that the .44 is really .424 inches? Why don't they just call it a .42 Mag?
It's actually .429
But anyway, this is one of those artifacts of the evolution from black powder guns loaded with separate components to the adoption of metallic cartridges.
In the days of percussion black powder revolvers, their calibers were specified as something reasonably close to their bore diameter. The barrel for a Colt 1860 Army or Remington 1858 .44 revolver started with a hole drilled to a diameter somewhere close to .440". After that, it would be rifled and the groove diameter would be somewhere around .455", but the lands would remain around .440". These would be loaded with lead balls around .455" or a little more, to get a god bore seal. All was well and the names made sense.
When metallic cartridges got invented and people started converting the cap-and-ball guns to use cartridges, the simplest way to do that was to drill the charge holes in the existing cylinders all the way through. This left you with a revolver with chambers that were around .450" all the way through. You'd then load the cartridge with a heel based bullet (as still seen on the .22 long rifle today, the bullet is the same diameter as the case, except at the base whre it narrows to fit inside the case) the same size as the round ball you would have used before.
Heel based bullets though, needed their lubrication grooves on the larger portion of the bullet, that lay outside the case. This meant they were messy to handle, the lube could be rubbed off, or collect dirt. To solve those issues, they moved to non-heel based bullets that were smaller than the outside diameter of the case for their whole length, so the lube grooves could be located on the portion of the bullet covered by the case.
Unfortunately, these smaller diameter bullets didn't properly fit the full diameter of the bore, and solutions of using hollow-based soft lead bullets to swell and better fir the bore didn't work all that well. So they started making guns with smaller bore to fit the new, inside-lubricated rounds. They still called what had been based on a .44 case a .44, despite the fact that the smaller bullet was now only .429. The same thing happened to make .38s really use a .358" diameter.