I have found myself in a position, as a DM, that is causing me some discomfort. What do I do when, in a specific situation, a PC's back-story and skill proficiency do not match? This has occurred to me on several occasions, but here is the most recent example:

One PC is a long-time, heavy drinker. Another rarely touches alcohol. The two have decided to have a drinking contest and they want to roll endurance checks to see who wins. However, the long-time drinker has a zero in endurance and the teetotaler has an eight. It seems to me, in this specific situation, their endurance rolls should be adjusted to accommodate their back-story, but I don't know how to adjudicate such an adjustment in a fair way. 

To be clear, I'm looking for ideas regarding this **type** of problem, not just ideas for the drinking contest scenario. With that in mind, here's another situation, off the top of my head. 

One PC grew up on the coast and has spent their entire life on, and around, boats. Another is from inland and has never been in a boat before. Now the two are going to row a boat, but the seafarer has a much lower athletics score than the other PC. Shouldn't he/she actually have an advantage in rowing against someone who has never touched an oar, despite having a lower athletics score? There's knowledge, experience, and muscle-memory that go into the task outside of just "Here's how athletic I am," right? 

The question is: have any of you experienced these specific situations where the back-story and skill proficiency don't match up and, if so, how have you dealt with them in a fair and consistent way? Do I just let the PCs roll and say, *Despite the fact that PC X has never (blank)ed before, they take to it readily...* and *Despite the fact that PC Y has been (blank)ing their whole life, they suck at it...*? 

For some reason, that seems lame. It completely disregards the back-stories my PCs took a lot of time to create and that are meant to be a real part of their character. 

So, I guess it's a question of realistic storytelling vs. mechanics, and I'm at a loss. Any advice appreciated.