Normally a glazed porcelain tile has a relatively durable glaze and is chip resistant. Since the chipping isn't occurring at the edges with lippage, it makes me wonder if those chips were present when the tile was installed? Check to see if any chips are filled with grout which would indicate they were chipped when installed.
The tile won't chip unless it is subjected to some type of force such as dropping something heavy or sharp on the tile surface. I have seen where some types of floor vacuums can impact the floor causing chips. Although it seems more likely the tile has been abused in some way to cause chips, there are tests that can be performed on the tile to determine how chip resistant it is. If the hardness between the body of the tile and the glaze is greater than 2 calibrations per the MOHs' hardness scale then it is supposedly more susceptible to chipping.
The good news is that tiles can be replaced. As long as you have some of the same tile that you have kept as attic stock or can buy more of it in the same shade lot, then you can replace the tile. Often the replaced grout joint might not match well at first because the original grout is maybe dirty or the new grout is from a different manufacturing lot or the conditions in which the grout is installed are different that will make it appear different. Eventually after maintaining the floor for awhile the grout blends in. Good luck.