Are you going to tell us that what we see in mid February matters more than what we see on Sunday? If a player doesn't perform on the field, you're damn right I'm going to think he's not that good, and if they do perform I will think they are good.
Players can get better or get worse, when they get better it's usually when they're younger and their understanding of the NFL is becoming better, adjusting to the speed of the game, etc. When they get worse, they're usually older and declining, players like Peyton Manning mask that by being so incredibly smart and understands the game better than everyone on the other side of the ball. Tom Brady is physically declining, to a MUCH lesser extent, but he is still able to mask that by being smarter than every defender on the field.
The combine numbers have nothing to do with how they grasp the game.
The "draft experts" like Mayock and Kiper/McShay are always saying the combine and pro days don't really matter, the reason every other mediot on NFL Network and ESPN back it like it's a holy book is because their networks air it and need to inflate the value of it to the public, keeping an audience and making money. There are plenty of people who think they can predict the first round of the draft by watching the combine.
If it was all that valuable than the Jets would have the next Randy Moss when they drafted Stephen Hill and the Oakland Raiders would be the most dominant team in the NFL being led by 7x NFL MVP Jamarcus Russel and his all-pro cast of 4.2/4.3 Wide Receivers.