Cooldown Timers VS. In game Time

So let me start off by saying that I play a lot of , and because of this, I am usually very conscious of the in game time. With Shaco, as most of you know, a normal strategy is to start laying down boxes at 55 seconds at either Gromp or Buffs. With 5% CDR in masteries only, his boxes have a cool down of 15.2 seconds. This would mean that laying down 4 boxes immediately upon cool down would be 60.8 seconds correct? Well, while laying them down at buffs (particularly enemy's Buffs hehe...), I've seen that I can start laying them down at 1 minute and finish 4 boxes at around 1:50. I have no clue why this is and it doesn't really work well with my math. So, this leads me to the conclusion that the in game time somehow is different from the cool down time. Or that the tooltip on Shaco's W is a little off. The only other champions you can really see this with is with his Saplings.