The hollow is deeper than the steel in front of it, which becomes convex.
Well, yeah, that's just what happens.
Maybe it was made that way originally, but considering how many butcher knives, hunting knives, and pocket knives I've seen this exact same thing happen to (including several of my own that have gotten that way through extensive use), I don't consider my explanation too unlikely either.
By the time you sharpen some serious steel off the end of the blade, so it's lost like a half inch or more of width, you start getting up into the thicker steel. If you want to keep the edge thin enough to cut well, you have to start sharpening/regrinding the sides of the blade as well, thereby erasing the original hollow grind.
Now, before we go off on a tangent here, I've seen folks suggest that sometimes grinds like this were done intentionally so you could keep the tip thick for heavy chopping & piercing, while leaving the edge thinner near the handle for whittling & such. That
sounds like a nice idea. In fact, I thought it was a darn good idea & made a great big bowie with such a grind. But then real world use showed me it wasn't a good idea at all. By design, the rear portion of the edge is too weak to withstand the heavy work. So the first time I struck a little too far back, or even chopped a big limb so that most of the edge is making contact, that thinner portion of the edge got some big ripples going well up into the main grind.
So I doubt this would be the goal with such a design, and if the rear hollow ground portion of the original was stout enough to withstand the heavy work, then there wouldn't really be a reason/need to make the tip area any thicker either.