Yes - 2nd and later round offers are the ones they haven't been taken up in the first round so if you don't tell them your are taking the first round offer they assume that if is available for offer to someone else in the second round and you may miss out altogether waiting for a later round offer.
It works like this:
1st round - 100 places offered at a cut-off of 88 - 96 people accept that offer meaning 4 places are available for offers in the 2nd round.
2nd round - 4 places offered and 4 places accepted - course full - no third round offers (and the people who didn't accept the 1st round offer are also out of that course).