The only way I can see that being true is that his kill rate as a tanker was far enough below his kill rate as a scrapper that he couldn't defeat enemies fast enough to prevent his health dropping faster than it would on his scrapper.
Just to make some numbers up: if on a tanker you are 20% more resistant to damage than on a scrapper, and kill foes at 80% of the speed, you incoming damage is going to be 3% higher on the tanker, over time.
Hypothetically possible on paper, not possible in a practical sense with Invuln if you take Invincibility in both cases.
This reminds me, one of the things players (people in general, really) were not good at was intuitively extrapolating performance, because changes that are quantitatively easy to extrapolate could have qualitative differences not easy to extrapolate. One analysis I did involved looking at sets like Willpower and Invulnerability that had scaling mitigation. You might think, because the numbers were basically the same, that Willpower scaled the same for Tankers as Scrappers, with the obvious difference that Tankers had higher health. And in a numerical sense, it did. But in a really important qualitative sense, the set scaled differently for Tankers and Scrappers. If you analyzed average critter damage as a function of archetype health and factored in Willpower's non-regenerative mitigation, an interesting fact emerged. The amount of extra regen you got for each target you added around a Scrapper was, under most normal circumstances, less than the damage that critter would deal. But for tankers, the reverse was true: the regen provided was slightly higher than the damage each critter would do. That means if you were a Willpower tanker, herding made you *more* survivable. If you were a Scrapper, herding made you *less* survivable, unless you had enough invention slotting to increase your overall mitigation high enough. Which if you think about it, is kind of what you would want to happen if you could make it happen as a designer. Purely accidental in the case of Willpower, mind you.
Invuln, and invincibility in particular, scaled differently. Under the same analysis conditions, on Tankers each target you added reduced survivability, to a point. Around six or seven targets, it leveled off. Meaning, as you herded up more minions around you, you'd see more and more incoming damage, up to about six. After that, herding more had no effect: the defensive increase roughly matched the additional incoming damage, so you could tank ten about as well as six, at least approximately.
I always found that to be interesting. Nobody else seemed to, but that's ok.
