The "run diskstats" function invokes iostat with only the -d option, which causes iostat to only show the I/O utilization averaged over the uptime of the machine. The -w option must be used in order to get the I/O utilization averaged over the specified period. We'll likely want to change the script to run iostat -d -w 5 and add some logic to suppress the I/O utilization averaged over the uptime of the machine.

# OSX: Simulate the iostat timestamp.
elif iostat -d > /dev/null 2>&1; then
    while [ 1 ]
    do
        date +'%m/%d/%Y %I:%M:%S %p' >> mongo-diskstats
        iostat -d >> mongo-diskstats
        printf "\n" >> mongo-diskstats
        sleep 5
    done

The first statistics that are printed are averaged over the system uptime. To get information about the current activity, a suitable wait time should be specified, so that the subsequent sets of printed statistics will be averaged over that time.

https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man8/iostat.8.html