The amount of time shown here on the forum has nothing to do with how long a forum member has been mystery shopping. It represents how long ago a forum member registered with the forum. Anyone with less than 20 or so posts shows up as "new participant" even if it has been 3 or 4 years since he registered with the forum. Some forum members register but never post, which is what goldrunner has done. goldrunner registered here 8 years ago but has only posted 6 times, so he shows up as "new participant" even though he is not actually new to the forum.
As for ValG who has been shopping for 2+ years, her screen name shows 6 months because she only registered with the forum 09/14/2015. The forum only counts the amount of time as a registered forum member, regardless of how long a shopper has been shopping. The count isn't completely accurate. It is automated so it moves up a specified times. For example, it shows 250 posts after one reaches 250 and doesn't change until a poster reaches 500. It shows 6 months after a shopper reaches 6 months and doesn't change until either 9 months or a year.
Best not to stress over minutia.