Midi Key Limits - Split Keyboard

In the Library, in the Piano section, there is a "Piano & Upright Bass" sound that is actually two instruments grouped with non-overlapping key limits, so you get Bass sound for the left hand and Piano for the right hand. All good here.

However, when you are playing on either side and cross that "split point" over to the other sound, you still get the sound you are coming from for a few keys (by a fourth to either side, to be exact). It's as though the key limit is "smart" and is moving automatically, expanding the key limit of either instrument over the other. HOW DO YOU ACHIEVE THAT EFFECT? When I manually split my keyboard to any sounds, the key limit is just fixed for all sounds.

I can change the sounds and their key limits within that "Piano and Upright Bass" template and still get the desired effect, but I want to know how do you get that. Also, I would like to expand the range of that effect (maybe an octave to either side, not just a fourth).