Looks like that is the case, intentional change of frequency.

From the site:
The tones are spaced at 1 Hz intervals, with ten tones per CD track, and the layout is shown below under Track Contents. At most frequencies you can hear the transition from one tone to the next, but you can also use the CD player's time readout if needed. For example, Track 10 plays 100 through 109 Hz, so if the time readout says 23 seconds that means you're 3 seconds into the 102 Hz tone. If you get lost, just rewind to the start of the current track and resume from there.