Is Julia so off re roast leg of lamb
There are a lot of variables here:


Oven temperature: As you mention, this definitely can be plenty off, but honestly most newer ovens are all right and meat isn't the most sensitive thing to baking temperature. You'd probably have had other issue before if it were the cause here.
Meat weight/shape: Bigger means longer cooking times. The weight is a good first approximation, but the shape matters too: if the recipe was based on a flatter piece of meat, it'd have cooked faster.
The starting temperature: If the recipe was based on room temperature meat, but yours was chilled or previously frozen, it'll certainly take longer.


I'd guess your issue was more than just oven temperature, but it's hard to guess whether it's weight/shape or starting temperature or both.

In any case, as everyone seems to already be aware, the only real way to be sure is a meat thermometer. You're never really going to be able to find recipes that match your exact situation well enough to avoid this kind of thing otherwise.