When you pre-register a domain name with a registrar (essentially an approved reseller) they are not actually reserving it for you at that time with the domain registry but simply taking note that you would like that domain and adding it to a list for the registrar to try and automatically purchase as soon as the domain registry makes the new gTLD available in the General Availability (GA) phase for registration, at which point anyone can register domains.
One of the reasons it is not possible for a registrar to guarantee you will get the domain you pre-register, is that multiple registrars may have taken pre-registration orders for the same domain name, and in which case the first registrar to get their order in with the domain registry will be successful for their customer.
In some cases the domain registry will accept lists of all pre-registration domains from registrars just prior to opening the General Availability phase, and if more than one person or business is trying to acquire a particular name they may put it to auction with the interested parties with the highest bidder winning the name, or they may award the names on a first come first serve basis given the date and time of the pre-registration.
The method followed is not consistent between domain registries so I would not be able to clarify further without knowing the specific gTLD you are referring to here.
If this is important to you it is always worth reading from the domain registry's website what their process is for the Pre-Registration phase and how they allocate their domains in the event of multiple parties trying to register the same name.