There are many factors that figure into a first impression. It is possible to meet someone for the first time when they are having a bad day, don't feel well or are simply just very nervous. As well, some types of personalities need time to evaluate and warm up to others before feeling comfortable enough to really be themselves. First impressions, therefore, are often inaccurate reflections of an individual's true character.
First impressions can be very deceiving. When meeting someone for the first time, they tend to put on an act for whoever they are meeting. It's not necessarily intentional. Some people are very shy and may come off as stuck up. Some people are talkative and may want to get to quickly know the person, so they may come off as annoying. You never truly can make a judgment call about a person until you know them fairly well.
There's a saying that "all that glitters is not gold". The first impressions that we usually make of the of the people we meet is a result of their appearances, clothing, manner and content of a conversation and other such insignificant factors. Little do we ask ourselves if these few things are enough to decide upon whether or not the person is deceptive. One impression is not enough to judge a person , just the way one test is not enough to test the caliber of a student, one impression is simply too inadequate.