Did 25k on the bike, iPhone says 7.8 k
Cycling doesn't trigger the 'step' recognition the way actual steps do. 

The subsystem responsible for the step sensor in fact does a great deal of work to filter out anything that isn't an actual step. 

So the fact that some cycle moves make it through could actually be considered a flaw. 

What actual cycling apps do instead is:


use the location service (e.g. GPS) of the phone,
merge this with optional data from other sensors (like wheel speed or cadence),
optionally feed the resulting cycle metrics into the Health app. 


This is why you can possibly find cycling data in someone's Health database – apps can use it to store their own sensor data or metrics. But so far (early 2016) iPhones don't directly feature built-in cycling sensors.