Sorry, typo! That should have read Jackson Co GA. Picked this info up from early GA and TN censuses where parents stated place of birth; then, supported from Mason family researchers. The place of birth shifts in the censuses after William was adopted. His wife is also listed variously as b. in PA, DEL, and TN.
Apparently the family migrated from Byberry PA through the Carolinas to GA near the time of William's birth then back up to TN (obituary articles cite Williamson TN as birthplace of William), then on to IL, etc. A very typical migration pattern across the various DUNGAN lines. Perhaps he was born just before they left, enroute or she went back home to Mom to give birth. Who knows! When the census data, family data, and biographical references disagree, I include all these possibilities in my full entries!