How the audience does not notice voice difference when "singing-voice" dubbing? [closed]

How the audience does not notice voice difference when professional singer, with totally different voice, performs musical number and then lip synchronized to the actual actor?

For example, Marnie Nixon was "singing-voice" of many popular actresses in musicals, but how people don't notice difference in voice?

Only logical explanation is that Marnie Nixon had superhuman ability to exactly copy other people's voices? Which I highly doubt...