Pianos are never really tuned to equal temperament. Look up "stretch tuning", which is something that will sound like it's equal temperament when played with other instruments, even though a lot of the fundamentals are actually off. It is also possible to do the same trick relative to some non-equal temperament, which is what piano tuners almost always did before electronic tuners came along (they may have *thought* they were using equal temperament but objective measurements said different). In idioms that center ona few keys - Cape Breton music, for example - the piano will usually be tuned to make the thirds for those keys as pure as possible.

Whereas guitars have to be equally tempered because of the layout of the frets. Which is why guitars all sound shite.