Firstbeat Bodyguard 3 measurement device that is used in the Firstbeat Life measurements have different step measurement methology compared to sports watches and activity bracelets which is why the results can have significant differencies.
In watches the step calculation is based on wrist movements. Detection of steps requires that the wrist moves during the activity, so the device also determines hand movements as steps.
The Bodyguard 3 calculates steps based on the acceleration data measured by the device. The measurement of the steps starts when the analysis identifies a longer continuous walking with a clear step rhythm, meaning that the device doesn't calculate for example individual steps taken while standing. As a result, the number of steps in the Firstbeat Life measurement may be less than the actual number of steps.