Hi Kent,
I pulled open a webench design with your specs and am seeing the same thing. In fact, this 5V supply is added for any output other than 5Vo. Even at 4.9Vo and 5.1Vo, this additional supply is added implying there is zero margin on this input which is unrealistic. See 'BOOTSTRAPPING' section on pg. 8 of the LM2621 Datasheet:
I have also included a few reference designs similar to your application showing BOOT powered from 3.3V:
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