I don't understand why the data table is so difficult to decipher.

If there are earthquakes in it listed at 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, and 20W longitude then there is a good chance that the expected earthquake will occur somewhere in that range, say at 15W for example.

It can be thought of as basically being a probability table for longitude locations. Sometimes that method produces good results. At other times the results are not as good.