This can be the case because, in a sense, most people have an inner knowing and awareness they are ill-prepared for life, so they suspect this of others as well and may rightly question someone who appears overconfident and who may be overreaching in claiming credit for what they may have been a part of but not the major player, and so on. People vary in their attitudes and feelings about challenging others or questioning their accomplishments and, in effect, their self-worth, and so on. It makes many people uncomfortable to even have that responsibility, whereas others delight in having a measure of power and control as a kind of gatekeeper, if only as a routine functionary.
So this gets into aspects of personality traits and leanings, but it is certainly the case that many, if not most people at times, feel themselves to be an imposter and this will be the case in a new setting where they lack experience in something that they wish to take on as a new role. Even if it is related to their field of expertise but something different, there will be misgivings until they are in the saddle, so to speak, long enough to become comfortable and feel truly competent and effective to the level they can take pride that they are mastering the new challenge. It is common for people to feel uncertain in a new job for as long as a year and sometimes longer, depending on the demands of the position, before they really get their sea legs, so to speak.
So during that start-up and integration phase, they must look the part and act the part to the greatest extent they can to keep up appearances, satisfy their supervisors that they were a good choice for the position and are meeting expectations simply for the sake of survival, and at some point looking for a wage increase or a promotion when they have gained a level of security. So again, there may be a varying level of posturing involved in the attempt to look good even though they might be making frequent errors. But if they are sincere and willing to work in earnest, and especially if there is good enough communication they can ask for help, they will make their way and survive at the expense of feeling uncomfortable in the stress of their fears about being found out and unmasked as a pretender, in a sense.