In regards to salary variances from one ranking to the next - if it is foul play, blame the school or blame the alumni - but probably not fair to blame them both.
My understanding is that many of these rankings make the actual data available to other schools to check for consistency.
IE is definitely known for entrepreneurship, so if one school were to be skewed abnormally high, a few highly successful entrepreneurs coupled with low response rate could potentially do the trick.
As a graduate from the University of Alabama, I remember the average starting salary of undergrads in the business school doubling from one year to the next - a football player who graduated from the b school got drafted and went pro making millions. Obviously median would need more than one person to be affected, but one team with a million dollar project could certainly impact things.
also, being an entrepreneur means your salary is a little more of a grey area than receiving a corporate pay stub. For example, one might think - my new company made 200,000 USD profit last year so I'll report that as my salary (even though I'm reinvesting most of it to keep the business growing). Not saying that's right, I'm just thinking out lout as to the nature of the results...