Exaggerating by half an inch to claim six feet tall really isn't in the same ballpark as just adding three extra inches to your height.A lot of guys who are 5'11 or 5'10 will just say 6'ft or slightly more as they feel their height starting with a '5' is looked down on by some.
It is really not uncommon at all.
These generalization have a significant amount of truth to them:
Guys lie about height.
Girls lie about weight.
You have to wonder about the mental state of somebody who adds three inches when they are already fairly tall.