I think there are a few possible factors:

- different packages of the same medication have different doses per pill. So you might read one package that says to take two pills, then later you buy a different package of the same painkiller that says one pill, but you're lazy, and don't read the label for this, and assume it is the same as last time, so you end up doubling the dose.

- For ibuprofen, the label often says if one dose isn't sufficient, then you can take another pill (with a maximum number in 24 hours). People may assume this is the case for Tylenol as well, but I don't think it is.

- people may read the size of the dose, but skip over the warning about the maximum you can take in a 24 hour period.

- The directions are in small hard to read text, which makes all of the above points worse.

Not that any of those are a good excuse not to read the label, but, well, people make bad decisions all the time.