No, if you look at WOL, and buffs gained, there are 3 different Windsong buffs that will show up with 3 different uptimes, for each of the 3 possible stat buffs. For example http://www.worldoflogs.com/reports/r...?s=5378&e=5833. You have to add the 3 together to get the total uptime of one of the three buffs.
It's entirely possible that 25 man uptime is significantly higher than 10 man uptime because you have a lot more things ticking on a lot more people at once, causing it to proc sooner after it comes off ICD.