ADS transient simulation with snp blocks

Dear Friends,
I am simulating 2-cascaded snp block with transient simulator in ADS and then using s-parameter simulation I convert the 2-cascaded block to a single block and then run the same transient simulation for this block.
The problem is that the simulation results are not the same in these 2 cases.
Would you please help me with my problem.
I guess transient simulator calculates the impulse response of each block seperately anf then convolve them together,maybe this is the cause.
Any idea would be appreciated.