The reason for that is that arctan (which is inverse function of tan) gives 2 values of angles between 0 and 2pi but we can go below 0 and above 2pi radians as much as we like (for example there is angle -10000pi or +3000000pi) up to infinity which means there are infinite answers.
It is important to remember that arctan gives infinite numbers of answer but function tan gives only 1 answer.
