It's two fold. First, injuries are completely unpredictable. Secondly, durability doesn't necessarily translate. But you obviously grade a more durable player higher. It's not an exact science, just a common sense approach to risk management. Drafting players is nothing but a series of risks, you just try to do your best to mitigate the risks as much as possible. I don't know how many players I've seen that stayed beat up in college and were labeled injury prone that went on to almost never miss games in the NFL. Conversely, I've seen many that never missed time in college that couldn't even get their career started in the NFL because they were always injured. The ability to stay healthy playing this game boils down to good fortune for the most part.
Although there's obviously players with certain frames that are just fragile and less durable than others. However, players that have the toughness to play hurt and with injuries are what you covet. There's an old saying among football coaches that the most important ability you can have is availability.
True story.....Bear Bryant once dragged a broken leg up and down the field for 4 quarters against rival Tennessee. He wasn't even supposed to play, he was in the locker room on crutches prior to the game. Now this is a different era of football, but you still want players with that kind of dawg in 'em....they're just hard to find. Especially in the NFL when they start cashing those big paychecks. Look and injury reports of NFL teams week in and week out and the players not suiting up. Sore knee, hurt ankle, bruised ribs, broken thumb nail, etc. It's ridiculous.
One reason I have such a high opinion of Cam Robinson is because despite his technical flaws, they just don't come any tougher than that kid. He played the first 2 months of last season through injuries that would've kept 99% of players out. But he never missed a game through it, and it's why he struggled so much for the first part of the season last year. Once he got completely healthy, he played dominant football for the latter half of the season all the way through the playoffs and national championship. You know you can depend on this guy.
Injuries, routing bumps and bruises, and legitimate medical risks all fit into different categories. They all should be weighed differently.