Some day when you have some time go to the TL, and search for each position and each skill, say a skill of average and above. And note the number of players that are found (if you find 1000, you may need to set some age limits).
For example, if you were searching for PF with JR of average or better you might not find too many, since JR doesn't really cause a player to be classified as a PF, and might actually cause the player to be a SG if the JS is also good and OD is at least tolerable. But you will find that PF will have a good JS.
So to be classified as a PF, a player may need to be good at a lot of skills, but not so good at some of them that he is classified as C or SG.
It is easier to train other positions since they need fewer skills. And it is possible that the person who plays PF in games might not need the same skills that cause him to be classified as a PF. A club team in D.IV might be able to get by with a PF who is classified as a C, and has some JS. This might be especially true if the team has a really strong pure C, or has good outside shooters and uses the PF as more of defensive and rebounding player.