[BC] Article in RW Workbench--Question?

[Jerry Mathis]

Am I the only one who doesn't understand this?
 
Referring to the Workbench article in the 2-1-2010 edition of Radio World. Our friend Tom O describes a method of adjusting a cassette deck motor (tape) speed using an audio generator and an oscilloscope. The procedure involves recording and then playing back a constant-frequency tone, and adjusting the motor speed until the two tones "sync up" on the oscilloscope.
 
I fail to understand how this would work, because if the tape speed is the same for both the recording and the playback, comparing the tone frequency of the audio oscillator and the tape playback will show no difference, irregardless of what the tape speed actually is, whether it's correct, or fast, or slow.
 
Tom, maybe you can elucidate? Or did they not explain this right in the RW article?

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Jerry Mathis