Why signal changes rise/fall time outside the subsystem block?

I have a subsystem that has a direct look up table in it. the output of this look up table is the upper one in the attached fig. Then through an output port, this signal is the output of this subsystem block. But when i put a scope at the output of the block of the same signal, it is very different, as shown in the lower plot. the rise/fall time of the original signal is 6e-7s, the same as the stepsize, but the lower one has 12e-6s. When the pulses get narrow, the outputs become triangles. Anyone has an opinion on this?

Thanks,