your hcg levels are going up :) , they dont usually match up so to speak, they have a guide and thats all it is a guide, as long as they keep going up you are fine, they say they can detecta heartbeat from around six weeks, but there has been many a lady on this site, that have been worried sick as there was no heartbeat detected at 6 wks and have gone back for another scan a week or so later, to see bub nice and strong :)
The first time I heard my babys heartbeat I was 7weeks
generally somewhere between 6 weeks and 8 weeks