How accurate are nutrition label / facts / info for restaurants & fast food joints?

I’m curious about the accuracy of nutrition info for places you eat out at. I’m wondering how much variation is there between the posted menu’s and what you really are eating in.

Jack in The Box’s website says a beef taco is 157 calories… how did they measure this? Is it routinely reviewed or compared to real life store versions of the taco?

One big question that comes to mind is variation between the product tested/measured vs what you typically would get at a store. For instance, for nutrition info testing, does a McDonalds Big Mac use a bare minimum amount of sauce, cooked extra dry/grease pressed out, etc to keep their fat/calorie numbers down?

vs

Is the real life prepared Big Mac loaded with more sauce and grease to keep the served product tasty… and filled with way more calories/fat than I am lead to believe by reading the nutrition label?