If exist a skill called stamina,there is a reason.The players lose performance when they play so much time...the two players that play 48 minutes against you plays against two players that play 40 minutes aswell,so while the playes of both teams lose performance,the differential remain basically the same,and your guards are much weaker than your adversaries's guards,as the boxscore shows
Players with mediocre or average Od will give you that kind of rating,nothing strange here
Where a player has many fouls his defence is less effective,so probably your PF have an higher shooting percentage against a weaker opposition