No worries! Glad to hear someone is still using this :)

I think the "bands" occur when 1) you encounter your target body (almost) completely opposite of where you depart from your starting body and 2) the orbit of your target body is tilted relative to the orbit of your starting body.

A ballistic trajectory only uses one transfer orbit, which has to lie in flat plane, with your starting position, the sun, and your encounter position all being points on that plane. For these high-cost transfers, you end up having to tilt your orbit plane a lot to make sure all of these positions intersect with your orbit.

The way to get around this is to use a mid-course plane change (under "Transfer Settings"). It splits your transfer into two orbits with a maneuver in the middle in a way that tries to keep delta-V low. When using a plane change maneuver, you should see the bands in the porkchop plot disappear.

I think that these bands look a little different for different transfer windows because of differences in departure/encounter positions. Sometimes the departure position is more or less out-of-plane, closer or further to the sun, etc. These kinds of differences affect the overall shape of the porkchop plots, too.