The Domain Name registration is done in a real time and it happens lot a time that you search a Domain Name and the next moment it is booked by someone else. There can be instances of Registrars or Sub-registrars booking the domains on the basis of your search criteria and then offer the same domain at a higher or premium pricing. The act like this
is illegal and you can very well take action against such registrars by reporting the same to the ICANN. Alternatively, there can also be the instance of someone genuinely booking the domain searched by you earlier. Remember, The Domain Name System is Universal and any person can book domain sitting at any corner of the world in real time.