Boxrec, Wikipedia, and even Top Rank's own website have Pacquiao recorded as winning only 6 world titles... not 8.
Quote from Top Rank's website...count'em:
"He has won world titles at 112, 122, 130, 135, 147, and 154 pounds."
WBC Flyweight World Champion (112 lbs)
IBF Junior Featherweight World Champion (122 lbs)
WBC Super Featherweight World Champion (130 lbs)
WBC Lightweight World Champion (135 lbs)
WBO Welterweight World Champion (147 lbs)
WBC Super Welterweight World Champion (154 lbs)
A) The Ring title is not an official world title.
B) The IBO Jr. Welterweight is a Regional Minor Title.
Chalk it up as the Guinness not knowing much about boxing.
Don't be surprised if he's removed from the Guinness due to a misinformed error.
Also, considering the title won at a catch weight against Cotto, and the vacant title won at a catch weight against a guy who wasn't even ranked at 154 (Margarito) it seems delusional for informed boxing fans to get excited this so-called achievement.