Opening a channel for the first time has a ton of stuff that must be completed that once they are in the channel doesn't have to be redone for each sample.
They built an auto plant near our city when I was younger. They built it in a field near the main highway. It took 3 years to get the first car out of that plant. Now would you think it would take 3 years to get the 2nd car out of the plant?? Of course it wouldn't. Same concept here. More gravel faster for the next sample due to less setting up the channel and equipment.