Yeah, City can't guarantee Champion's League Final tickets (if we ever actually get there!) as it's UEFA's call how many of the tickets for the final each participating club actually gets. Barcelona and Juventus were both allocated just 25% of the tickets on sale for last season's Champion's League Final, that equated to just shy of 20,000 tickets each. According to the UEFA Website the rest were allocated as follows:-
Each applicant was asked to complete the online application form for a maximum of two tickets and entered the ticket lottery, regardless of the time of submission during the application period.
Applicants will be informed by email by 10 April as to whether they have been successful or not in the lottery. Also, applicants will be able to check the status of their orders via the 2015 UEFA Champions League final ticket portal. Tickets will be dispatched by registered post no later than the end of April.
So, 50% of the tickets (minus those given directly to corporate sponsors etc) are sold via an open lottery to whoever choses to apply for them.