Hmm, why might you underestimate velocity?
You might have slightly under-measured your distance (perhaps neglecting the transverse displacement had a little effect--but I don't think that would cause the magnitude of errors you are seeing).
More likely is that your scope might not have a quick enough response, so you over-measured the travel time. Or your laser might take a moment to start. Same result.
Hard to say really.
Also, what is the horizontal coordinate of that graph measuring? If you just made 4 measurements of speed, it isn't appropriate to put them on a line graph like that. On the other hand, if the horizontal component measures something that could effect your speed of light measurement, that might tell you something.
Edit: So the horizontal axis is measuring the distance you used to measure speed? Now that DOES tell us something useful. See how your error is big for smaller distances and smaller for larger distances? That definitely indicates that some sort of time lag is responsible for the error. And, given several measurements, you might be able to correct for it.
To get your velocity, you divided distance by measured time.
But actual velocity = distance / (measured time - delay time)
There are 2 unknowns in that equation: actual velocity (what you want to measure) and delay time. But given a second measurement, you can get a second equation and do some algebra to solve for the delay. Then take the delay into account when you calculate the speed of light. With multiple measurements at different distances, you can check and see if the method produces good results.
Please do this, and make another graph and show us how it works out.