Every side has a belly section, so it's not a chromexcel thing, it's just a hide selection thing. Good chromexcel from the better parts of a side doesn't do that. There are a lot of makers using entire sides of second quality chromexcel that are pretty much ALL loose grain, but that's more common with carry goods, not boots.
Basically when you see loose grain on a boot, it's a cost cutting measure. In order to be as efficient as possible, the pressure is always on to sneak portions of belly into products. Leather sells by the foot, so the more belly you throw away, the higher the pricetag goes.
I've yet to see any White's or Vibergs with obvious loose grain, in Chromexcel or any other leather.