Actually, it doesn't even make sense. If 60% decays in 3 months, shouldn't 60% of the remaining 40% decay in 6 months (for a total of 84%). And then in another 6 months 84% of that would change which would give us 98% or "1 - (2/5)^4" of the web changed if 60% changed in 3 months. Unless there's something very funky going on, the rate of decay should stay constant!