Can hcg and scan results differ so much?

I did a blood test 4 wks ago to confirm my pregnancy when I missed my menses. Back then the hcg results was 5 wks gestation. My first day of last menses is at 15 jan. I just went to the gynae today (1st visit) hoping that we will be able to hear heartbeat (since now should be around 8-9 wks based on blood test result). However when scanned, it’s around 5 wks and foetal pole can be seen. I’m scheduled to go back 2 wks later to see if it’s growing and if heartbeat can be heard to confirm this is a viable pregnancy. Question is.. if US scan and hcg test can differ so much? Can’t help by worrying that it is not growing as it should.. #advicepls