What is drift?

Gatling (Open-Source)
1

Here is the setup:
setUp(
standardUser.inject(heavisideUsers(usersCount).over(sessionLength / 2)))
.pauses(uniformPausesPlusOrMinusPercentage(10))
.assertions(
global.responseTime.mean.lessThan(50),
global.responseTime.max.between(50, 500),
global.successfulRequests.count.greaterThan(1500),
global.allRequests.percent.is(100))

usersCount is 200 and of those about 10% only execute second action after 40 seconds delay after first action