VAN persie is a number 9 and suarez isn't Luis has been great this season but van persie has been very consistant down the years and Luis has been in and out a bit
I guess you're right, although RVP does have a couple bad years his goal totals were down to appearances as he was often injured. Suarez does seem to be improving year on year from the below stats.
RVP:
2011-2012 - 38 apps 1 sub - Goals 30
2010-2011 - 25 apps 6 sub - Goals 18
2009-2010 - 16 apps 2 sub - Goals 9
2008-2009 - 28 apps 4 sub - Goals 11
2007-2008 - 15 apps 2 sub - Goals 7
2006-2007 - 22 apps 5 sub - Goals 11
2005-2006 - 24 apps 11 sub - Goals 5
Suarez:
2012-2013 - 20 apps 0 sub - Goals 15
2011-2012 - 31 apps 2 sub - Goals 11
2010-2011 - 13 apps 1 sub - Goals 4