First off, I think you mean *38* weeks, not 28.
Second of all, the reason it varies so much is that 1) babies vary at when they're ready and more importantly, 2) due dates are just estimations and are not precise.
After 37 weeks you are considered full term. However, you want to wait to 40 if you can to have the baby as the brain still develops and it has advantages to wait vs., say, induce early. But again, you can't necessarily know if it truly is "early" or your due date wasn't accurate.
In my case, I had an EDD of March 31 based on my LMP. I knew that was off based on my charting of ovulation and when we had sex. My revised DD was given to me after my first u/s early on (at when I "should" have been about 7weeks) and moved to 4/11/13 instead. Now, I am measuring a bit ahead of the 4/11/13 DD. So if I have the baby at 3/31--is that even "early"? Or is it just a sign they don't know when the baby comes?
As to your NICU question, it depends on the baby & how (s)he is doing. Most born a little early are fine. (I was born 2 weeks early and was fine and didn't need a NICU stay.) Flip side, a friend of mine had a rough delivery, emergency-C, and the LO had to stay in the NICU for a week and a half--and he was born 4 days late. So the due date isn't as big an indicator as how the baby does in delivery, how big (s)he is, any other complications, etc. By 2 weeks, I wouldn't expect any NICU stay unless something else is going on.