This is an amazing bit of advice that every home brewer needs to understand. IBU only tells part of the story, and you have to understand that there are other factors that go into perceived bitterness. Many of your darker beers have higher IBU values, but the non-fermentable sugar and the other roast flavors counter the hop bitterness. Adjuncts like lactose can also smooth out some of the sharper hop notes (again, non-fermentable sugars). I found a guide that shows ibu ranges for a bunch of styles and you can see that a lot of heavier beers are rather high in IBU even though you’d never call the style “bitter” or “hoppy”