Can anyone explain why a player's attributes may change slightly after you acquire them? Speed goes from 92 to 88, FOV goes from 48 to 52, pass rush goes from 79 to 82, etc.? Not talking about a big boom or bust after training camp, but I have noticed ratings going up or down after trading for a player or signing a free agent. Just wondering if there is something with the coaches that may give you a better idea of a player's "true" ratings when he's on your squad as opposed to when he's on another team. Thanks for any input.

You basically answered your own questions.

Speed/acceleration will increase/decrease based on weight changes. Players lose/gain weight by trending toward the ideal weight for their position (actual position, unrelated to depth chart). Picking up a player and moving them to a different position has an impact pretty quickly on weight.

Mutable technical skills change from game to game over time based on the hidden trend variable, coach skills (requires being on a team), etc. Picking up a player means that the coaches' skills are being taken into consideration for future ratings.

The way you're describing it, it probably has more to do with my coaching staff. I do look for highly qualified coaches (besides the head coach's specific playbooks). I know physical attributes like speed/acceleration/strength change with a position swap weight gain (RB to FB, TE to RG, etc.), but I'd figure that current levels of skills like route running or B&R avoid (not future potential) would stay the same regardless of the team.

I don't look at player skill levels on a game to game basis, just see what happens at training camp and work on the assumption it will be at that level for the entire season. I know rookies develop quicker with more experience, but I figure that a fifth year WR has most likely reached whatever ceiling he has and won't suddenly boom out of nowhere.

Go to the page for a player that's been around for a few seasons and click on the progress tab. You can see that slow incremental changes happen game by game, but TC (especially the first) is the biggest jump/fall.