Sure, go and ask the HR. They should be able to explain.Although it is strange that your co-workers are willing to let you know what their checks are. most people will not do this.
I am not sure they have to give you advanced notice. Although until recently I didn t realize companies could cut your salary when you had done nothing wrong.As for an explanation - I would ask HR what is up. Could be a mistake.
Go to the HR dept and ask them why your check was less. It s possible there was a mistake made. If not, they can give you an explanation of what the decrease was.