I noticed it today. It's apparently been an issue for a month or longer. (there are posts in the Blaze and Charge forums as well).
this is from last month.
https://community.fitbit.com/t5/Android-App/Hourly-activity-tracker/m-p/1934482
They are working on it.
One person's theory is that it is related to steps in the first minute of the hour not "counting" in the hourly goal (In the app. The devices are counting it fine). https://community.fitbit.com/t5/Blaze/250-steps-per-hour-wrong/m-p/1850618#U1850618
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