why do measurments vary from standing up to laying down?

could someone expalin to me why they vary?
see ive been hitting 6 3/4'' for a while but i always measure standing up , so last night i did my measurments regularly and the decided to see how id measure laying down. so i did and i could barely hit 6 1/2!!! could any one explain why that is or if they experience the same thing??>

?

Depends on your posture, the location of your pelvis to be exact. When you tuck in your pelvis properly you get a proper measurement. When you're lying down your ass sinks into the mattres and so does your penis.

Yup

Measure sitting down. This will be the longest and most accurate measurement.