Around 50 million years ago, in contrast, oxygen levels in the atmosphere measured 23 percent--2 percent higher than today.
Oxygen levels have been higher than 23 percent.
Reason:  Never says 23% is the highest.  Why:  23% is the highest mentioned in the context.
( z1 have-degree :ARG1 ( z2 level :mod ( z3 oxygen ) ) :ARG2 ( z4 high :ARG1 z2 ) :ARG3 ( z5 more ) :ARG4 ( z6 percentage-entity :value 23 ) )