If the wheel size is correct, the difference in speed between Strava GPS tracking on the phone and cycle computer boils down to the following:
- Strava doesn't take gradients into account when calculating speed. The actual distances on a gradient and speeds on a gradient are slightly greater than what Strava says.
- Strava speeds can be higher than cycle computer speeds if you spent a fair chunk of time on red lights, because Strava doesn't take that into account when calculating average speed. Your cheap non-GPS cycle computer doesn't care though.
- While post-ride it doesn't matter because it's going to recalculate it correctly, watching the speedometer during the ride if you're riding anywhere which impedes GPS signals (just trees will do that), it's going to show a significantly lower than real speed, or even zero (eg. in tunnels). Strava will also do funny things when you go through tunnels, like conclude you went over the mountain if there's a segment like that. Whooops.
- If you brake hard enough the wheel doesn't have proper traction anymore, or jump, or do anything where the front wheel isn't gripping to the ground properly - there's going to be a small difference on that account. Not going to be significant, though.

Some differences are normal and just an artifact of using a different method of measuring speed.