Probably just dumb luck. Bethesda games are chock full of it because of how they handle loot. Loot is scaled depending on where you get it from (i.e. chests, barrels, dragons, people, etc.) and what level you are. This leads to loot being in places where it would make NO SENSE to be, like Imperial armor in a Dwarven ruin. Mods like Morrowloot fix this to how Bethesda used to do it: gear is still scaled based off of your level, but it's in places where it would make sense to find it (like Stoarmcloak armor in a Stormcloak fort). It's probably a similar thing with the dragon bones: the game couldn't properly scale the amount of bones to your level. It was probably a stronger dragon like a Blood Dragon judging by the amount.