MSFHQ
1st round has more open slots and schools will give out acceptances to students knowing that only a % will actually show up. It is the same thing as overbooking a flight. As the rounds go further the schools have a better idea of how many students will actually show up on day one and the number of open slots diminishes. They will use these last spots to round out the class (bump up the GMAT or GPA average, increase diversity, accept top applicants, etc). Some programs will have a ton of open slots to fill and these later rounds will actually be easier. The issue is you don't know which schools are like this so it is better to assume none are like this.
The MSFs / MoMs I'm applying to are USC, University of Virginia (McIntire), University of Michigan, Duke, University of Texas at Austin, and Washington Univ in St Louis. Not sure if you know how many open slots these programs have in the 2nd round. I know they are all pretty good programs but in terms of how they go about giving out acceptances I have no idea.