i guess he was stating its more odd than anything. have you actually "connected" your phone to it "handoff" or what ever its called. try turning that off. i can only send imessages off my mac as my phone isn't connected that way.
though bizzarely, with one of my mates it auto sms's a lot of the time. no idea why. might have something to do with them changing settings, imessage not recognising them etc