The rating is based on actual performance, so it will vary from game to game. It also has some dependence on the opposition, if your so-so shooter goes up against a great defender, he might look like a lousy shooter in that game. There will be some variation with tactics used. If your offense is Look Inside, your C and PF will shoot more and may get a boost in their ratings, while your guards may have lowered ratings.
It may especially vary for players who don't play much of a game. This might simply be an artifact of having less data on which to base ratings.
Over several games ratings can give you an idea where your weaker areas are. It can also help you scout your opponents.
I use someone who is classified as a C as a PF. BB has a shortage of players classified as PF and PG (assuming you'd expect 20% at each position). This may be due to a weakness in the classification system (which has been tweaked), or because there simply are not as many players who have developed the skills for these positions.
Defense simply isn't as sexy as offense, and passing is even less so. So it is more fun to train offense than defense. And C don't need as many skills, they need IS, ID, and Reb, with perhaps SB and Pass. A PF needs to add a JS, and perhaps some OD. If you do play a C at PF, use the one with a better JS, even if it is inept-mediocre or better OD as the PF, unless he is also a much better IS and ID.
If you look at the lineup screen (or the training screen), it gives you an idea on where players play on offense. As in soccer the actual position at any time may vary, but this gives a general idea. The C and PF play closer to the basket, while the SF, SG, and PG playing more on the periphery. At times, the SF may come in deeper and the PF further out on the side. And any player may penetrate toward the basket and receive a pass for a layup or shorter shot.
The forwards and guards may also play more symmetrically (which is why they are called forwards and guards). In that case, the center plays in the center of the defense or offense, in front of the basket rather than directly underneath it. The forwards play more to the side, and perhaps a bit deeper towards the corners, while the guards play out front. This is more a traditional classification, but is the position that the NBA shows in box scores (ie. 1 C, 2 F, and 2 G; rather than C, PF, SF, SG, PG).
All positions are both offensive and defensive. With only 5 players on the court, each player has to cover an opponent. There are differences in how inside and outside players play defemse. An outside defender wants to try and force the ball to the periphery, and prevent passes to the inside. He also wants to keep his man from getting a good shot. He has to run around more. Inside defenders concentrate more on establishing good position, or preventing his man from getting good position near to the basket. Rather than stopping a pass from being thrown, he tries to stop it from being caught, or playing close enough defense that the pass won't even be attempted. Since he plays closer to the basket he should position himself to get rebounds (and to keep his man from getting rebounds).