macOS: Piano keys sound 2 semitones lower than actual pitch

I am using Apollo Twin X audio interface on my MacBook Pro, and the sample rates of audio interface and engine are both set to 48,000 Hz.

However, the pitch of the piano keyboard located at the left of the piano roll sounds 2 semitones lower than actual pitch. For example, when I click C on the keyboard, it sounds like B♭. The pitch of the voices, however, are normal.

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Huh, good ear, I never would have noticed.