A query on Ks value

[zeam]

Hi there,
Recently,I'm reading a paper published on PNAS,and after identification of syntenic blocks using SynMap,it said: "the synonymous substitution rate of individual gene pairs do not pemit genes to be unambiguously classified as othologs or ancient homoeologs.The median synonymous substution rate of all gene pairs in a syntenic block between maize and sorghum can be used to classify syntenic blocks of 12 or more genes unambiguously as orthologs or homoeologs,however." The title of the paper is Differentiation of the maize subgenomes by genome dominance and both ancient and ongoing gene loss.

I did not understand what the" median synonymous substution rate of all gene pairs in a syntenic block "? Can somebody fimilar with this explain it to me ? Thanks.
Zeam