Yeah, like I said, it's doing 'something'! Can't find anything concrete except for general consumer Apple talk:
"The Process Buffer Range: setting determines how large a buffer Logic sets aside for its mixing engine. As with the I/O Buffer Size, smaller settings decrease latency, but increase CPU load. The default setting of Medium is usually fine, but if you experience frequent System Overloads, then try setting it to Large."
So there's a mixing buffer, and i/o buffer.
Well, if Apple won't give specifics... I'll find them out
I have a neato utility which let's me see what my UAD plugins are running at as far as a buffer latency. If I use a 128 buffer in Logic, I can still see that the UAD plugs are running at 1088 samples. Changing the i/o buffer in logic has no effect on the plugins... however, changing the process buffer range does indeed change it. Setting it to large increases the buffer to a whopping 2112 samples.
So yes, Logic maintains a pseudo-hidden mixing buffer, but no, it doesn't change it on playback, it's always there.
Now we've all learned something today!