You could spend a lifetime watching TV sitcoms and not realize that the same weird things keep happening from show to show over time. Maybe that's because we don't watch that many sitcoms at a time, or we don't compare our favorites from the 1970s to our favorites from the '10s. Did you ever notice how new infants in a sitcom family suddenly become old enough to deliver lines (or at least look cute) the next season? In Full House, Uncle Jesse's twins grew quickly, while their cousin Michelle did not.
Modern Family provides more evidence of a vast baby-swapping conspiracy when Cam and Mitchell celebrate their daughter's second birthday ... only to mention that she's three a few months later. Clearly, they're speeding up the rate of her birthdays so as to avoid arousing suspicions when they inevitably switch her (which they did).
On the other hand, the baby swaps in Growing Pains and Family Ties are so laughably obvious that they seem to imply this business isn't even underground. Chrissy Seaver blossoms from basically a newborn child to a 5-year-old girl in the space of one season ...
That's just one weird thing about sitcoms that happens over and over. You might notice one thing when it happens in your favorite show, but you probably never realized that it's a TV thing in show after show, over decades. Read the rest of the list of weirdness in TV sitcoms at Cracked.