Ultrasound said i was 6 weeks with a yolk sac however my blood work showed hcg levels at only 894 and that put me at 5 weeks difference why is that?

Too early to tell: Additional information on the accurate measurements of your mean gestational sac diameter and the size of your yolk sac performed by a skilled certified sonographer and the dates is needed. HCG levels reflect trophoblastic growth; they can vary greatly and may unfortunately be inaccurate /unreliable.A repeat repeat blood betaHCG test and a repeat sonogram may offer you an answer to your question.