My 2 cents that I should know better than to add..
OOTP is a computer game it is not real life. The scouts are "bags of numbers" trying to create an illusion of a person that is a scout, he's not real. Being a computer game there will be shortcomings and places you have to "add to the story" to make it real in your mind.
In this case I'd create in my mind the following..
Many scouts, coaches, and other players have always thought "Nick" would be a good starter. Hell, Nick always knew he could do it but he never got the chance. The organization he was part of didn't see him that way, had a ton of P's they thought were better, thought he'd pay off more being a top line closer and never had him work on a starter's routine. Take your pick. In this scenario Nick never had a chance to show or even work on stamina due to the team's "know it all coaches".
Then one winter Nick goes to a pitching guru thinking with work he could convince the team in ST that he should be given a chance. Hell if not he'll got to FA and find a team that will give him a shot. The Guru is amazed at how Nick flies through his program. Other Ps go to the guru too and word leaks out. Scouts start paying attention, he throws at the teams facilities over the winter to real batters, and the team starts to see the light and "talks him up". Why not? If he's great it won't be a secret for long, it could generate excitement and ticket sales.
But alas, OOTP is just a game and isn't going to leak this info to you slowly over time. You have to take a "leap of faith" that the "bags of numbers" uh, I mean scouts have done their work and this has become the consensus view of the scouting world.
Or.......... You can go to commish mode and simply edit his endurance back to what it was and move on. You have already shown that you're not going to find an answer here that will satisfy you. So next best thing is to do whatever you need to to make your baseball world real to you.
