The difference is that Dispatchers.Default is limited to the number of CPU cores (with a minimum of 2) so only N (where N == cpu cores) tasks can run in parallel in this dispatcher.
On the IO dispatcher there are by default 64 threads, so there could be up to 64 parallel tasks running on that dispatcher.
The idea is that the IO dispatcher spends a lot of time waiting (IO blocked), while the Default dispatcher is intended for CPU intensive tasks, where there is little or no sleep.