Per round you'll only get 1 offer at most and that will be the highest preference you've been offered. You could get offers from every single preference but only the highest one will be offered to you.
In your situation (to illustrate what happens), if you got both med and sci, you'd only be offered med in round 1 because science would be lower in your preference list.
You would have to then accept your first round offer and hope you get sci 2nd round if science was the one you really wanted. In the case of your med vs. science scenario, I think your view of med first and science 2nd is probably the wiser choice.
This is why it is really important that the preferences are ordered correctly based on your desire for specific courses and not based on ability to get in etc. because these choices can have an impact on what you get offered, keeping in mind most offers happen in round 1. In addition, most courses have caps / limits and if all places are filled after round 1 (really depends on the course) then you may run into a situation where the course is full and you won't be offered a place in round 2.