Skin effect applies only to AC so we can forget about that here (and the conductors are in contact- we don't have a Litz(endraht) wire).
Now, on DC only- you have one particular chart that shows stranded wires have higher resistance but there are other charts. For example:
In this case it appears to be related to the type of stranding, with "C" concentric stranding showing a lower resistance than solid. There is not a strong correlation with the actual areas either, rather the stranding method appears to be the main factor.
So presumably there is some interaction between the contact between the strands, the strand configuration etc. that causes an increase in resistance in the twisted styles.
Here are the actual areas in mm^2 calculated from the total strands/strand size using the formula for AWG area which should be good to about 6 significant digits.
The formula for resistance also yields somewhat different results than the chart above as well. There are a number of assumptions (temperature, copper conductivity, which is a function of annealed vs. work-hardened etc) as can be seen in the references 7. from Wiki.