First I think this might be useful to add some comments
Then, why is there an operation using the worst_status?


I understand what you're trying to do:

if there is `N` check with `n` that are critical this will return `N + n`(assuming no 'unknown' status)
if there is `N` check with `n` that are warning this will return `n`(assuming no 'unknown' nor 'critical' status)
if there is `N` check with `n` that are unknown this will return `2N + n`
else if all are ok it will return `0`

Plus: 
The `value_threshold_warning`is set to `STATUS_WARNING = 1` (it's a non sense to sue the `STATUS_WARNING` constant here)
The `value_threshold_critical`is set to `nb_checks + 1 = N + 1`


IMO that's kind of weird. At least not very intuitive, is there a better way to do it.