I bet this is a leftover from the Prophet 08... perhaps even a bug that never got fixed on the Prophet 08, and that DSI decided not to fix (or talk about) since fixing it would no doubt create chaos with any and all programs made prior to it's discovery...
I tested this a bit, and if you set up a program with LFO1 set to square wave, and route this to pitch and set the amount to maximum, then if you were to get the same modulation from the Mod Matrix, then the amount knob would have to be set to only 32... this hints that Mod Matrix modulation amount set to the same as in the LFO secions are actually four times as "strong".
Since the Prophet 08 also had Mod Matrix slots, and the REV2 is backward compatible with it, the Prophet 08 must also have had this peculiarity ... if it's an inherited bug or intended, only DSI knows.
But all in all, I think it's nice because you then have a way to make more subtle modulations via the LFO section which is especially suitable for vibrato as you were talking about here.
I actually did not know this... so thanks for bringing it up
What I'm now wondering though is, if the LFO Amount is at 25% of the actual modulation, and the ModMatrix at 100%... or if it's 100% for the LFO section, and 400% for the ModMatrix.