You also have to remember the writers don't care too much about continuity. It's a comedy and not a serious drama. There are a lot of examples to cite, but I recall an episode where the Simpson family is banned from the state of Florida. In a future episode they travel down to Florida because Edna Krabappel is nominated for a teaching award. Not to mention a handful of episodes where we look into the future. I think Lisa was supposed to be in college and get married in 2010.
Also...
Spoiler
Show
...wasn't there an episode involving Homer building a dog house and a swear jar where Ned shaved his mustache? Like in the episode mentioned in the first post Ned grew it to cover up a scar, but when he shaved it no scar.