two different due dates , should I worry?

im a bit worried about baby. lately i have noticed that both my last period date and the size of baby dates are two different due dates. from my last period it says im thirteen weeks and five days but the size of baby says im ten weeks and four days. im a first time mom , so i don't know if i should worry about this or if it's normal.

if im thirteen weeks then my due date is july 23rd

if im ten weeks then my due date is august 7th.