So, I've been re-watching the episode where Ash caught Taillow, trying to figure out why does it resist electricity so much which should be weak to. But... that episode didn't reveal anything, to make matters worse, none of the episodes did. There were multiple situations in Hoenn where Ash's Taillow/Swellow is shown resisting electric-type moves and even the characters mentioned that it's weak to them. The thunder armor thing was just the final nail to the coffin, proving once and for all that Swellow resists electricity. Okay, so maybe Ash had a special Swellow, but the thing that annoys me the most is not that it's resistant, but because it was never explained - why.