You have to consider that the kidneys never stop making urine, as in continuously, so unless you emptied just moments earlier, there will always be something filling your bladder. The more you have had to drink, the more urine that will be created, but there will always be some. However, the bladder isn't like a water cup, it expands and contracts and things depend on your position, so it's possible that urine is collecting in a part that doesn't immediately reach the discharge point.
Don't discount the power of the brain to hold on to certain controls. I'd say 9 times out of 10 the issue is brain induced. Now that I've been working on my incontinence, I can wet in any position, even laying on my belly in bed and sitting while driving. It took a long time to be able to wet while driving but it's recently started to happen.
Harley's need to be needed makes her much more human and less a force of baby-hungry madness. Still, she either needs therapy (willingly) or someone who shares that with her (willingly) if she's going to see her happy ending, I imagine.
And that last line was felt right in the ego!