A Space Marine lascannon fires only once every five seconds, but deals very high damage spikes when it does. Two Eldar fusion guns fire almost continuously and do their damage in small increments instead. Both end up dealing similar DPS (33 vs. 36) over time.
A Tactical Squad with two bolters and one plasma gun deals a low amount of constant damage from the bolters, broken by sudden high damage spikes from the plasma gun. Mark of Tzeentch CSM with three inferno bolters deal a steady stream of moderately high damage. The average DPS output ends up in the similar range again (60 vs. 68).