Well this is more gut feel than definite fact, but here goes:
The filter tracking driven from the combination of octave switching and key pressed.
The Oscillator tracking is driven by octave switching + key pressed + osc frequency.
So if you don't adjust the filter cutoff there is an extra step in the oscillator chain.
If you change the octave of the oscillators using only the osc frequency controls, the filters will still be tracking at the setting of the octave switch. Whereas if you change octaves with the octave switch both the oscillators and the filter will change their tracking.