ee savings bonds rates and yields
We have EE savings bonds that were acquired at different times over the years. We use the savings bond wizard program from treasury direct to keep the inventory. I am confused by the rates and yields it reports, as they seem like they must be computed differently from how similar terms are computed for CDs. For example, we have bonds, purchased at different times, all showing rates of 4%, but showing various yields for various lots (e.g. yields of 5.3, 5.37, 5.45, 5.55 and 5.6). We have one group that shows a rate of 4.15 but a yield of 4.13. So clearly something different is happening here than just the results of compounding of the interest rate to get the yield as with the rate/yield quotes on CDs. Can someone explain or point me to a source to clarify? Thanks for any help you can offer.