I think it is normal. It can't be a solid straight line, as the transfer rate should be spot on the same the whole time. When copying a file over a network, there is overhead (the bits before and after a data packet (identifiers and crc etc). The switches or network cards in between can suffer, and have a mille second delay if it is a continuous stream of data, and then it cause what you see (rubber-necking) in peak hour traffic in JHB or CPT.
Also Switches and network cards check and match mac addresses, and control the flow. Say you have a 2nd device on the network, and it also try to-do something, then it takes time for the switch to keep up with regulation and passing on of traffic.
So you end up with data with a graph like above.
QoS would sort most of this out, and give you a smoother throughput, as it will give the traffic preference to the other traffic on the network.
