gprof too low cumulative time relative to actual running time

I am profiling a C++ application with gprof by running it for over 5 minutes. gprof shows the cumulative time as if it only ran for 60 seconds.

I have read much about its limitations and none seem to apply to my case; the C++ application is very straight-forward (no recursions, no i/o), just a bunch of classes with methods and calls from main.

I am trying to understand what could cause such a low cumulative time relative to the actual running time.