Can't figure out my due date!

Heather
I know most babies grow at different rates but I'm confused on what my due date really is. 
Based off my last cycle my doctor says my due date is November 24th, even though he is off by a day if he really went by my cycle but that's what he says it is
My first sonogram that wasn't with him said baby was measuring to have my due date November 21st, which is the date that I go by because I know ultrasounds predict due dates better than the last cycle
My last sonogram was 3 days ago at 17w1d along(based off my due date I use) but she's measuring for due date to be November 19th. Is it normal at this far along for baby to already measure big or do we really just don't know what the due date really is?