Gap bet theween concrete and soil

After digging next to my concrete patio, to see the extent of a rodent den, I discovered (besides that the rodent den was not too large) that there is about a 1" +/- gap between the bottom of the concrete and the top of the soil.

From the hole I dug I could insert a tape measure about 6ft deep under the patio. I do not know if it is localized here, or if this condition exists all around the patio.

Any idea what caused this? Erosion? Poor compact of soil before pouring?

And more importantly, what needs to be done to fix it?

Normally the patio is just used for a dining table and chairs, but I have been eyeing a stone grill kit from Menards, which is approx 4000lbs in stone. I do not want to get this and make it crack. The patio is approx 1.5-2 years old.

Thanks for the help!