This questions is both easy and very hard to answer. The hard answer would need hours to go through, so lets try the abridged version:
All companies have different underwriting criteria, or in other words take a different view on what makes a high or low risk. They take into account all manner of different snippetts of info (like being married or not) into account, but the crux of it they compare your info and compare it with the info that others insured with them (and others to a certain extent) and how that info maps to a claims history.
So putting it simply:
Company A & B both have a broad cross section of the population insured with them, however Company A has had a large number of single females aged 35-41who smoke & drive VW beetles make a claim with them in the last 6 months, whereas Company B has had no claims from this group, but has had to pay out oodles due to the bad driving of divorced men aged 59-70 who live in Milton Keynes and drive Volvos. Obviously Company A will therefore quote more for the chain-smoking female bug fanatic, whereas company B has it in for the male divorcee from Bedfordshire.
As to the question as to quote validity:
This is primarily to ensure that if the change there underwriting criteria, then they can reflect this by requoting. If they havent then they will give a very similar quotation the second time around, which you will likely be happy with.
Fiamold aka Dave