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Why do restriction enzymes tend to have an even number of bases in their recognition site?

When reading my textbook I noticed that in all examples but one from eight the recognition site was an even number of bases.

I wondered if this was just a co-incidence, so I took the data from this site for over a thousand known recognition sites and put it into a spreadsheet (XLS uploaded here). The results are probably best summarised graphically:

restriction enzymes pie chart

restriction enzymes bar chart

Can anyone explain or suggest why it is more common that restriction enzymes recognition sites have an even number of bases?