Do they have a trait that increases it (if you're running mods, this is highly likely)? Scenario editor shenanigans? Used custom settings in the storyteller? Do they have a drill arm or other specialist body part you just didn't notice?

If you are playing a different difficulty then usual that effects mining yields. Just the baseline difficulties alone can make it up to 40% between runs. Steel has a baseline of 40 steel and pawn skill and difficulty both impacts what you actually get. 14 skill does increase yield past 100%. Yield also effected by sight and manipulation, ideology can boost it too. Some sort of mod like an alien race or gene that boosts mining yield could I'm sure get pretty crazy, according to the wiki yield caps at 125% from the mining yield stat though.

I think one of the VE mods has an inspiration that increases mining yields substantially.