It's been my experience that the people who use phones as their cycling computer get pretty inaccurate gps readings which leads to inaccurate distances and speeds, which is especially noticeable when they are out mountain biking through the trees. I've just noticed this over the past few years on strava. When some ridiculous gps route is shown, like having the rider pedal a mile and a half through a lake, it's usually because that rider is using a phone as a bike computer.
I personally get very accurate readings on my phone. I used to get less accurate readings on my older (6 yrs old at this point) phone. It just depends on the phone and how good the GPS receiver in it is. I think most recent smartphones have pretty good GPS modules now.