The rumor probably comes from the same person who says that dishonorable discharges are automatically upgraded to honorable six months after you get kicked out. Or from the same group of idiots who get married so they get "extra money". You know who I'm talking about.
The difference in run times could be several things. Change in weather (most likely), change in sleep patterns, change in eating habits (everybody who had to eat frozen sandwiches for 3 days this year will back me up on that) you will all experience your first few days at OCS and can affect how you run. I agree, I've been through the Physical Fitness and Body Composition order before and can't find this 2.8-3.2 thing either. If you just like to be safe, practice on a 3.2, then you'll always be fine on an actual PFT.