Minecart with TNT misbehaving through water

I was doing some redstone using Minecart with TNT to bomb a place. When the circuit was complete, I used it with unexpected results.

What I expected to happen was...:
When the Minecarts with TNT were toggled, they would take as much time as normal TNT between activation and explosion, and don't grief the circuit thanks to some walls of water.

What actually happened was...:
Minecarts with TNT blow much sooner after activation than normal TNT blocks. Also, while they go through water, their explosion still affects the other side of the water wall.

Steps to Reproduce:
Download the map I attached. It generates air layer flatland, with a 3x3 chunks grass platform for natural spawn not meaning death by void. Over those chunks, the circuit is set up. Use the button at the end of the white wool line.