It refers to people with the same experience with the system. For example, lets say you have two persons working with a system and have worked with it for two years. One of them is 10x as productive in that system than the other. Neither of them had experience with such systems prior to this, so both were equally productive when they started, but one of them grew much faster and is now 10x the other.
Of course if you took those two and put them to work at a new system both would have 0 productivity initially as they learn the new system, but as they grow the differences starts to appear and soon you have a very stark contrast again.
Of course if you took those two and put them to work at a new system both would have 0 productivity initially as they learn the new system, but as they grow the differences starts to appear and soon you have a very stark contrast again.