Hot water lines, causing a dew point. Seen it myself many times.
Internal plumbing leak.
Built on a natural spring.
Soil high on the perimeter, making it below grade.
It could just be random error. I have seen many slabs test 99% and a new hole new probe doesn't read 99 real close to original test.
The contractor put a sealer down prior to the epoxy surface coating.
Never had any resolution on the 99%. Tried switching out the probes but watched the humidity rise back to 99% by the next day. After the fact we cored the concrete adjacent to the probe installation to make sure the probes were at 40% depth, check. Just another floor moisture issue where the cost to find the conclusion just isn't worth it.