Many sites seem to be discussing the raging controversy over the true birthdate of the late Al Lewis. A consensus seems to be emerging that he was definitely born in 1923, and he began saying 1910 when he did The Munsters. He was playing the father of Yvonne DeCarlo, who was born in 1922 and either (a) Lewis was afraid someone would think he was too young for the part so he fibbed or (b) the studio thought it might bother someone and they asked him to lie. I'm not sure either would be a very good reason but his being younger than Ms. DeCarlo seems to have been part of the reason.
Actually, there are plenty of instances of actors playing parents where the real ages don't match up. Maureen Stapleton was the same age as Dick Van Dyke when she played his mother in Bye Bye Birdie, and I'm sure other examples will come to me later. There might even have been something colorful about Al Lewis being younger than the woman playing his daughter when they were both portraying vampires. Maybe it's that they feared it would call attention to Yvonne DeCarlo's age and she (or the studio) didn't want that.