Because they counted from December 1987 to December 2000, which is... 13 years indeed.

Hmm... Let's all count together here... 1988 (1), 1989 (2), 1990 (3), 1991 (4), 1992 (5), 1993 (6), 1994 (7), 1995 (8), 1996 (9), 1997 (10), 1998 (11), 1999 (12) 2000 (13). It seems to me that NOT 1987, but 1988 - 2000 equals 13 years. If it had been 1987 - 2000 it would be 14 years.

Continue this trend and you get -- 2001 (14), 2002 (15), 2003 (16), 2004 (17), 2005 (18), 2006 (19), 2007 (20), 2008 (21). I'm no mathmatician, but there it is in plain print that everyone can understand. The reason for this is that the year 1988 actaully = 0 (zero). Let's not forget that 0 really is a number and is a placeholder. So in my scale above... if you replace the number 1 next to the year 1988 with a zero (such as 1988 (0), 1989 (1), 1990 (2), etc, etc) you'd eventually get to 2008 (20). Afterall, doesn't 2008 - 1988 = 20?? But yes, the roots of the band with Billy and James may have began in 1987. However the first release was this in 1988.

http://www.spfc.org/songs-releases/discog.html?discog_id=137

And "I Fall" was also recorded in 1988 acording to this:

http://www.spfc.org/band/studio.html?session_id=2