Cut offs change every year depending on various factors such as no.of Candidates appearing for the exam, no.of Candidates qualified, top NEET score and difficulty of the exam. Allotments are done on the basis of merit and preference. Let's say for example, you have better rank than me but not enough for a seat in a GMC. But your first 10 choice Colleges have been only GMCs and then later you choose Deemed Universities or Dental Colleges. On the other hand, I choose all Deemed Universities for the first 15 choices and then opt for GMCs, I would get a seat in Deemed University even if my rank is higher than yours. Because for my rank, GMCs are difficult but Deemed Universities are an option. So it all depends on your choice filling. And mock allotment actually gives you a rough idea of how your options.