This is the way I understand it...
The reason coils/wicks get gunked-up with crud over time is because of the "caramelization" of various components of e-juice.
Things like flavorings, colorings, and sweeteners, are most likely the main culprits.
Well, it stands to reason that a fair amount of "caramelized" gunk might be released into your juice as well.
And I would imagine it might be heavier than regular juice, thereby tending to sit in the bottom.
So if you keep refilling it without ever cleaning it, this is what might be expected to happen over time.