Leads that qualify for a smart campaign run through the flow with a Change Data Value flow step, but some leads don't have a data value changed, while other flow steps still apply. It seems that those leads have just skipped the Change Data Value step and went right on to the next flow step.
If the new value for the Change Data Value is exactly the same as the current value in the lead record, Marketo will skip that flow step. There won't be an activity entry in the activity log or results tab because the action did not actually happen.
Say you had a field called Favorite Sport and a lead that already has the value of "Football" in that field on their record.
The flow step is Change Data Value: Attribute: Favorite Sport New Value: Football
In this example, a lead will run that through flow step, but since the value did not change Marketo will just move on to the next flow step.