Hi Kelly,
A HCG of 530 would be closer to 4 weeks than 5 weeks (on average - as Anthea said it varies a lot!). With your scan they have dated you as 6 weeks 4 days (they go off the average fro the scan). So I'd say you are actually just over 7 weeks pregnant, and the scan is measuring a bit behind (measurement error or slower starter) by just under a week.
As a note on the early scans, that is the date that will be used to officially calculate your due date, so work off that one. Both my early dating scans have been behind where I actually was (by almost a week), so I think it's pretty normal. All my later scans show baby being in line with my personal calculated date. When I had my son (on his official due date) his observed age was 41 weeks which is what I thought! Unless your 12 week scan is out by over a week they won't change your official due date.
According to the dating scan, my current pregnancy HCG was 170 at 3w2d and 600 at 3w5d, which don't agree with the dating scan at all (out by almost a week), but I've still been stuck with my dating scan due date. Was concerned that something was wrong when baby measured 'behind', but was just the inaccuracy of the scan.
Hope this helps!