clear blue test

Rebecca
Hi 
Can anyone explain this to me please.
My clear blue pregnancy test states 1-2 weeks pregnant but when I read up on it, sites say I am actually 3-4 weeks. Why is this?? 
Each site I put my last period in (17th Nov medium flow, 18th light flow & spotting on 19th) states I'm due around 27th Aug 2017.
Thanks x