OK, I can't help myself. Here's the difference:
The Airtap unit Xringer installed is a pre-engineered, somewhat drop-in unit. The Water heater fitting is made so you can assemble the whole shabang without much chance of anything bending very much. The Heat exchanger coil slides straight in, and can be positioned while the plumbing is somewhat dismantled. It has a swivel fitting, so assembling the plumbing presents no further twisting once the tubing is snaked into the vessel.
When HV23T made his heater, he snaked the exchanger coil through the rather large hole, leaving a few feet of tubing sticking out. He threaded these pigtails through the fitting, then tightened the fitting while the pigtails were still free to not spin the whole assembly inside the tank. If anything, the ends may have spiraled around each other a few revolutions. Once the through-wall fitting was tight, he then tightened the compression fittings for a waterproof seal.
The heat exchanger randy made is all one piece. Besides the wrangling of fitting the tubing through the bung hole, he had to twist the entire assembly multiple revolutions to tighten the through-wall fitting into the bung hole. Since copper is somewhat rigid, more so the larger the tubing diameter, when the exchanger contacted another surface inside the tank, it tried to crease rather than bend.
Rather than force it in and hope for the best, randy wisely decided to stop and re-bend his exchanger so it will fit. I would have done the same thing. Trial and error, man. That's what it's all about.
For what it's worth, I don't think it will make a whole lot of difference what shape the exchanger is, as long as it fits in the tank. Lanky snake or twisty slinky, as long as you put the thermostat down low in the tank it will sense cold inlet water arriving. Once the unit fires up, convection will mix the water pretty quickly.
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