there are many variables in measuring your cock that can give an entire inch or even more difference
method one:
get a hard erection and put the ruler against the pubic bone, let it rest on tip of dick and let it go
this is the lowest measure ment
method two
same but bend the upward curve into straightness
dick is longer here
method three:
same as 2 but press the ruler into pubic bone fat
difference between 1 and 3 is an inch or more