why I am getting this inconsistency ? (Knowing many factors can be the deciding point like number of clients, bandwidth per client, encryption algorithms, compression, packet size, network errors, QOS, latency, link utilisation and other network parameters)
Different intervals having different amount of data transferred: (eg: Interval: 3.01-4.01 is 0.00 Bytes)
This test was done from a client's laptop from Office Wireless Network (Iperf Client) to Data Centre Server over MPLS link (Iperf Server).
I have attached the output, Thanks and much appreciated.
I don't know anything about iPerf, but it seems to me like you've already answered your own question... Wireless performance is seldom the same from one minute to the next in the real world and there's any number of reasons why it changes.
If you wanted to get more insight the first thing I'd do is to repeat the test (as closely as possible) via the Wired LAN and check you have consistent results that make sense there first. If you don't get consistent LAN results, you will never get consistent WLAN results.