Missing twin epidemic?

I don't know how common this is elsewhere, but I'm always seeing people pushing double strollers along the streets of Cambridge that only have one child in them. What happened to the other kid? Maybe they go to different schools? Maybe one is home sick for the day? Maybe one is off in Switzerland at ski camp? The mind boggles. Sometimes I even wonder if there is another kid. Maybe rockin' the double-wide is how this little gangsta rolls...