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A little focus breathing, a little bit of the reality where zoom lenses are not always perfectly rated to their focal lengths, and a little bit of my testing imperfections where I was moving the tripod to test various focus distances for the different lenses.
For my uses, the focus breathing is a non-issue. There's a small range of scenarios where it would come into play, but generally, if an animal is letting me get close, the extra foot or whatever that I have to step forward to correct for breathing is not a problem, and a big chunk of the time I'm not able to get that close anyway. I think the focus breathing is something that looks really bad when you test it, and then you actually go out and shoot and find it's not nearly the problem you imagined.
For people pinching pennies, the 300mm + TC delivers a really excellent value proposition. Additionally, people who shoot exclusively at the long end or can carry multiple lenses are also getting a big win from the 300mm. The 80-400mm is pretty tremendous, though, and is a killer travel lens.