The National Weather Service figures we're too dumb to understand that the monsoon season runs through different dates from year to year. So now the monsoon season will start and stop on official dates every year. Of course, the actual monsoon may (and mostly likely will) extend before or after the official dates.
The rationale for this decision is so people will pay more attention and be prepared for the monsoon hazards if they know when it is coming officially rather than when it actually shows up.
You know, sunrise and sunset change every day. Maybe we should redefine sunrise as 6 am and sunset at 6 pm just to make it easier. All you would have to remember is that the sun rises at 6 am every day, except for those 363 days a year when it doesn't.