SPICE simulators adjust the time-step dynamically, in order to yield the best precision with the lowest simulation time. So you will have a smaller time-step when the signals are changing fast, and larger time-steps when things have settled.
A uniform time step would either result in poor accuracy (in case of large time-steps) or in long simulation time (in case of small time-steps).
What you usually define is the MAXIMUM time step, i.e. the time-step is adjusted dynamically, but is never larger than the value you indicate. If I'm not mistaken, HSIM this is done using the HSIMTAUMAX parameter.
Regards.