I have no logical explanation for why there is a dip in the rates between the 18 month and 24 month period. However, the percentage difference is de minimis, unless your principal is of epic and biblical proportions. My own bank does not do that, as far as I know. A lesson I learned long ago when I was a young lass was not to leave money in a CD for more than 3 years at a time. A lot can happen in the meantime and you may need the money after all.