It could be that the other devices pitch shift the input signal up (maybe by an octave) before it hits the tracking. As
@Fjord posted above, with the Axe III you can insert a Pitch block before the synth while using a bass guitar to pitch shift the signal up an octave before it hits the Synth block so it can track way down beyond the low B, in which case perhaps a future firmware update could just have this as a built in feature in the Synth block itself.
It would be great to know if a C4 style selectable tracking range would be possible. The C4 itself can even handle A0 even if it's not listed in it's not covered by it's tracking range