What if they're always 20-30 levels higher than you? There could be 3 types of dragons like one poster said, one being the highest, about 20+ higher than you... maybe the first type will always be 5-8 levels higher and the mid dragons will be 10-15. At the beginning of the game, they should make sure you at least encounter a dragon early in the game, like the mid one, then you'll have to run to safety, a town maybe, you shouldn't be able to kill it like in the trailer.
No idea about Alduin's level, but i'm tired of this leveling with you nonsense, these should be bosses essentially. Imagine at lvl 60, any of these dragons will be tough, but by then you'd have dragon shouts, but even with the strongest dragons it should still be a tough fight.