Question about Fandango (movie tickets)

I have a question about how the Fandango movie ticket service works, where tickets for popular movie can be purchased in advance for a slight surcharge in order to avoid long lines and/or “sold out” signs after driving to the theater:

Do the theaters that use this service give a certain percentage of the total available tickets to Fandango to presell? Alternatively, is it possible for an entire showing of a movie to be pre-sold days in advance via Fandango? If the former, is it possible for a particular showing at a particular theater that is sold out on Fandango to later have its status changed to “not sold out” due to Fandango receiving another allotment of tickets froms the theater. Sorry if this is an esoteric question.

At my local theatre, Fandango is only allowed to sell a certain percentage of the tickets for any given show. When we tried to buy ROTK tickets over the holiday, we got a message that they’d already sold all the tickets they could over the internet. We went to the theatre and got 9 tickets no problem.