Why is resting heart rate higher than the lowest value recorded in a given day?

[GBurn]

Each day I look over my heart rate graph and see values consistently lower than the value my fitbit provides as my "resting heart rate."  Why doesn't it just report the lowest value, or maybe the lowest value maintained for a period of time?  What is the Fitbit formula for computing the resting heart rate?

[Heather-S]

@GBurnFitbit estimates your heart rate based on your data from when you’re both awake and asleep to estimate your resting heart rate. Your heart rate likely drops below your resting heart rate when you are sleeping or being still and quiet, but being up and active during the day brings up your average slightly. 

My RHR is usually around 57. It looks like it goes as low as 48 at night when I'm sleeping, but I usually have intense workout in the morning and then when I'm working and doing stuff during the day, it probably hovers between 64-90 depending on what I'm doing. 

You can check out this help article on heart rate here and it may help as well.