Reports about who has been cast in a major role circulate long before anything is confirmed, and a large share turn out to be wrong. The pattern follows from how casting is actually conducted.
Secrecy creates the vacuum that rumour fills
Studios delay announcements to control the moment of reveal, and the gap between a decision being made and being announced can run to many months.
During that period a great many people know something, because casting involves agents, producers, schedulers and studios, and each of those has staff.
Demand for the information does not pause while the announcement is held, so partial knowledge circulates through informal channels instead of official ones.
Scheduling leaves visible traces
Productions have to book studio space, crew and locations, and those bookings involve enough people that they are difficult to keep entirely private.
An actor turning down other work, appearing in a particular city, or becoming unavailable for a known window is a signal that can be read from outside.
Much speculation is therefore inference from logistics rather than information from a source, which is why it is frequently directionally right and specifically wrong.
Auditions produce more names than roles
Serious consideration of several performers for one part means several sets of representatives, each with their own reasons to be optimistic.
Information about a screen test is accurate at the time it circulates, but it describes a stage in a process rather than an outcome.
By the time it reaches an audience the framing has usually hardened from consideration into confirmation, and the qualification has fallen away.
Denials carry less weight than the rumour
Studios routinely decline to confirm casting for reasons unrelated to whether it is true, so a denial is a weak signal in either direction.
Audiences learn this, and treat official statements as part of the negotiation rather than as settled fact, which keeps speculation alive after it has been addressed.
That dynamic means a rumour can persist for a long time without ever being resolved, until an announcement finally supersedes it.
Attention rewards being early more than being right
Casting speculation attracts sustained interest, and the reward for publishing first is far larger than the cost of a correction later.
Errors are quickly buried by newer material, while a scoop that proves accurate is remembered and cited, so the incentives run one way.
Studios have adapted by announcing more casting themselves and earlier than they once did, which narrows the window in which speculation can take hold.