Hi,

Babies are still babies, regardless of how they are fed. They don't *know* they are to be formula fed and so in the eyes of the adult world (and the side of the formula tin) they are to drink what the tin says and no more!) Sucking is everything for a baby. When they suck their release hormones like CCK which help them to relax and sleep, and endorphins which make them feel happy and content.
As Marushke says, all babies are different. Boy babies drink more (generally speaking) than girls - they grow fast that girls (that's why there are 2 sets of growth charts) and so they eat more to sustain this grown

So, yes, it's normal for bottle fed babies to comfort feed.