Different coffees have different caffeine levels.
Home roasted arabica may or may not have similar levels to a roast being used in a coffee shop.
Or, there might be robusta in the blend that will increase the caffeine content.
Different brewing methods can also change things up.
There was quite a discussion on this site regarding measuring caffeine levels some time ago.
So, given the starting point (a shot of coffee) is not really fixed, it makes it hard to have an accurate idea.