iperf test lost [SUM]

I run iperf to test bandwidth. I use parallel 15, window size 512k, test 20 minutes. I only get 222 [SUM], which is not enough. Why some of the [SUM] lost?

here are part of the report: from 1042 second to 1046 second, only get one [SUM] but all the other data are full.

[ 14] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[  4] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[  7] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[  3] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[ 12] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[  5] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[ 13] 1042.0-1043.0 sec   256 KBytes  2.10 Mbits/sec
[ 11] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[  9] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[ 16] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[  6] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[ 15] 1042.0-1043.0 sec   256 KBytes  2.10 Mbits/sec
[  8] 1042.0-1043.0 sec   128 KBytes  1.05 Mbits/sec
[ 17] 1042.0-1043.0 sec   256 KBytes  2.10 Mbits/sec
[SUM] 1042.0-1043.0 sec  2.12 MBytes  17.8 Mbits/sec
[  3] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[ 13] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[ 14] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[  9] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[ 12] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[  6] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[ 10] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[ 15] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[ 11] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[  3] 1044.0-1045.0 sec   128 KBytes  1.05 Mbits/sec
[  4] 1043.0-1044.0 sec   256 KBytes  2.10 Mbits/sec
[  4] 1044.0-1045.0 sec  0.00 Bytes  0.00 bits/sec
[ 17] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[ 17] 1044.0-1045.0 sec  0.00 Bytes  0.00 bits/sec
[  5] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[  5] 1044.0-1045.0 sec  0.00 Bytes  0.00 bits/sec
[ 14] 1044.0-1045.0 sec   256 KBytes  2.10 Mbits/sec
[  8] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[  8] 1044.0-1045.0 sec  0.00 Bytes  0.00 bits/sec
[ 10] 1044.0-1045.0 sec   128 KBytes  1.05 Mbits/sec
[ 12] 1044.0-1045.0 sec   128 KBytes  1.05 Mbits/sec
[ 13] 1044.0-1045.0 sec   256 KBytes  2.10 Mbits/sec
[ 15] 1044.0-1045.0 sec   128 KBytes  1.05 Mbits/sec
[  6] 1044.0-1045.0 sec   128 KBytes  1.05 Mbits/sec
[  7] 1043.0-1044.0 sec   256 KBytes  2.10 Mbits/sec
[  7] 1044.0-1045.0 sec  0.00 Bytes  0.00 bits/sec
[  9] 1044.0-1045.0 sec   256 KBytes  2.10 Mbits/sec
[ 11] 1044.0-1045.0 sec   128 KBytes  1.05 Mbits/sec
[  8] 1045.0-1046.0 sec   128 KBytes  1.05 Mbits/sec
[ 16] 1043.0-1044.0 sec   128 KBytes  1.05 Mbits/sec
[ 16] 1044.0-1045.0 sec  0.00 Bytes  0.00 bits/sec
[ 16] 1045.0-1046.0 sec  0.00 Bytes  0.00 bits/sec
[  3] 1045.0-1046.0 sec   128 KBytes  1.05 Mbits/sec
[  9] 1045.0-1046.0 sec   128 KBytes  1.05 Mbits/sec
[  5] 1045.0-1046.0 sec   256 KBytes  2.10 Mbits/sec
[ 13] 1045.0-1046.0 sec   128 KBytes  1.05 Mbits/sec
[  6] 1045.0-1046.0 sec   256 KBytes  2.10 Mbits/sec
[ 14] 1045.0-1046.0 sec   256 KBytes  2.10 Mbits/sec
[ 17] 1045.0-1046.0 sec   256 KBytes  2.10 Mbits/sec
[ 11] 1045.0-1046.0 sec   128 KBytes  1.05 Mbits/sec
[  7] 1045.0-1046.0 sec   128 KBytes  1.05 Mbits/sec
[ 12] 1045.0-1046.0 sec   256 KBytes  2.10 Mbits/sec
[  9] 1046.0-1047.0 sec   256 KBytes  2.10 Mbits/sec