Synchro is a mathematical model that approximates intersection operations and calculates delays and queues from a limited number of input values. The results are reasonably accurate for most conditions being analyzed. If the SimTraffic simulation associated with Sychro is run for a long enough period, it does appear to illustrate conditions not reflected in the Synchro output.

VISSIM is a simulation model that tracks individual vehicles through the constructed network based on a matrix of driver population parameters. My understanding of VISSIM's output data is it is compiled only for vehicles that have successfully exited the network. Data associated with vehicles trapped within the network by congestion doesn't make it into the output data.