Out of tune is another thing entirely.
As a fiddler I'm probably more out of tune (especially as the evening progresses) than a guitarist, who, given a decent guitar and careful tuning, will play fairly in equal temperament (which of course is equally OUT of tune in every key) all night. But I and others find that moving round the circle in a sing/play around, each time the performer changes the pitch changes, and, as we base our intonation relative to what is being played, have to retune the whole fiddle.
Whistles also vary a lot in pitch, but that's in the making of them and can't be helped. So it's better to ignore your tuner and tune to the pitch of the whistle, if there's only one there.