Cinder之前，sysbench IO测试和 sar io监控吞吐量为何不一致？

环境：vmware vsphere虚拟化 centos 6.3
磁盘：15K

sysbench测试如下：
[root@test data]# /tmp/sysbench/sysbench/sysbench --test=fileio --file-total-size=2G --file-test-mode=rndrw --max-time=180 --max-requests=100000000 --num-threads=16 --init-rng=on --file-num=16 --file-extra-flags=direct --file-fsync-freq=0 --file-block-size=16384 run
sysbench 0.5:  multi-threaded system evaluation benchmark

Running the test with following options:
Number of threads: 16
Random number generator seed is 0 and will be ignored

Extra file open flags: 3
16 files, 128Mb each
2Gb total file size
Block size 16Kb
Number of IO requests: 100000000
Read/Write ratio for combined random IO test: 1.50
Calling fsync() at the end of test, Enabled.
Using synchronous I/O mode
Doing random r/w test
Threads started!

Operations performed:  45200 reads, 30124 writes, 0 Other = 75324 Total
Read 706.25Mb  Written 470.69Mb  Total transferred 1.1494Gb  (6.5302Mb/sec)
  417.93 Requests/sec executed
吞吐量：(6.5302Mb/sec)             IOPS：417.93
General statistics:
total time:                         180.2311s
total number of events:             75324
total time taken by event execution: 2880.7254s
response time:
      min:                               0.23ms
      avg:                               38.24ms
      max:                               983.45ms
      approx.  95 percentile:          169.47ms

Threads fairness:
events (avg/stddev):          4707.7500/117.72
execution time (avg/stddev): 180.0453/0.05

sar 监控如下：
[root@test ~]# sar -b 1
Linux 2.6.32-279.el6.x86_64 (test.com)  2015年08月18日  _x86_64_       (2 CPU)
TPS：797.98 bread/s：25535.35 ~= 12Mbytes
15时07分56秒    tps    rtps    wtps bread/s bwrtn/s
15时07分57秒 797.98 797.98    0.00  25535.35    0.00
15时07分58秒 798.00 798.00    0.00  25536.00    0.00
15时07分59秒 812.12 812.12    0.00  25987.88    0.00
15时08分00秒 800.00 800.00    0.00  25600.00    0.00
15时08分01秒 780.20 780.20    0.00  24966.34    0.00

如上测试，sysbench的吞吐量为：6.5M，sar经过换算( 1block = 512bytes )为：12M。请高手指点...