By visits I presume they are actually referring to "hits", the black-sheep of Stats, the number for true "visits" would be lower than hits, I don't see the WTquoting the lower number, see this quote :
" Hits - The real Black Sheep in the family. The average website owner thinks that a hit means a visit but it is very different (see item 1). A Hit actually refers to the number of files downloaded on your site, this could include photos, graphics, etc. Picture the average web page, it has photos (each photo is a file and hence a hit) and lots of buttons (each button is a file and hence a hit). On average, each page will include 15 hits."