There are many surveys and studies which state various theories about girth size and blah, blah, blah….Besides the survey quoted by OP, there was a Lifestyle condom survey and Kinsey survey data, etc. I am not sure about the Lifestyle survey, but I know that Kinsey and many other surveys are self-conducted surveys which are not very accurate.
The problem with self-surveys is that we tend to visualize our girth as being bigger, and then as said by
WiL, we tend to round off to the next bigger number.
Nevertheless, one should remember that all the surveys are dependent on how a guy measures his penis, how accurately he measures his penis, and where he measures. A majority of the survey participants will mention their thickest measurement. No problem if it is in the wrong place.
Dear OP, do not take these surveys very seriously. Be happy with what you have.