homework and exercises - Analyzing Velocity vs time graph

So I did an experiment on conservation of momentum, where two carts with different mass start at rest and get accelerated and we measure the speed. What I observed was that after the acceleration, they started to decrease in velocity. I thought this was due to presence of frictional force, and I used regression line to get the "a" of friction. And here's what I got, and from here, I cannot seem to go further because the graph seems like it's contradictory. Shouldn't the slope of both of them be the same as a = ug? (where u is the coefficient of kinetic friction)