Why is the over speed warning coming on in FSX for the 737? need help!!!!?

Okay, in real life the max speed of the Boeing 737 is 509 kts, economical cruising speed is 430 kts and max cruising speed is 500 kts. So why the hell when I'm flying this aircraft in FSX the over speed warning alarm goes off right after 344 kts? WTF?!!!!! It's always like this and even with other aircraft the damn alarm goes off all the frigging time. What is going on?? Why is this happening?