Can you clarify which year group you are applying to OP ?
When you talk about re-applying in year 7, I assume you mean different schools from the original you applied to in yr6? and that you are talking about Shells/y9/13+ entry?
If that is the case, I was not aware that if you were not successful for your yr6 ISEB, you can sit for the same schools the next year, because ISEBs are age-adaptive. So, if a school is interested in you, you get waitlisted.
Yes, a lot of people sit on offers. Sometimes, till the very last minute.
Usually a school will have a deadline for paying the lump sum acceptance deposit, a lot of multiple offer candidates will release a space at this point. If they are on a waiting list to another school they desire more, they will keep their 2nd/3rd choice, and only drop it once they are quite sure what they are choosing. Some schools may ask waitlisted candidates to keep on sending in their school reports, usually end of the term. They may do this till end of yr8
My nephew was on a waiting list for a very popular school, his Head suggested he push an intense course for 2yrs on his favourite, but rare instrument that he was very good in (but he had only recently started it, and it was not that clear to his parents that he was particularly gifted in it). He actually got a music scholarship in yr8. So only then did he release two other firm offers, from his 2nd and 3rd choice. He kept his 3rd choice as they were still deliberating logistics.
What does your Headteacher say?