Yardage for any pattern is always an estimate, and while we do estimate a bit over what we used to help account for variables, there are many factors that affect how much yarn you will use throughout your project.
One of those variables is your row gauge; if your rows are more compact than the pattern row gauge (meaning: if you have more rows in 4 in/10 cm than the pattern recommends), you will undoubtedly use more yarn in your project. Usually, there's enough of a buffer to make it work, but not always.
Yardage between skeins varies slightly, as well, and it's always possible that we could end up with a skein or two that have more yardage or less yardage than what's listed.
Beyond that, any modifications you might make - such as adding a little length to the body or sleeves, or working more short rows, will consume more yarn as well.
Our rule of thumb is this: when in doubt, round up. It's always better to have a little more yarn than not enough.