Ive been wondering the same thing Tanya, as it sems some women get their dates changed with each scan. babies don't all grow in the same straight-line pattern - it makes no sense to continuously move the goal posts.
I have 2 dates. one based on my transfer (5/12) and one on an early scan (11/12). Since i was induced at 42 weeks las time the midwife agreed it made sense to go with the later date to give us more time.