In The Child (Season 2), Worf is called "lieutenant" and so is Geordie, but they have different pips (one has 1 full and 1 half pip, and the other 2). Why is this so?
The reason is that there are three forms of lieutenant - lieutenant junior grade, lieutenant and lieutenant commander (see images below). In this episode, Worf is Lieutenant (Junior grade) and Geordie is a full Lieutenant. An example of the Lieutenant commander rank is Data. For further reference, see this page.
Lieutenant (junior grade)