cbworkloadgen.exe does not set number item as in option -i

Install couchbase server 1.8.1-935 on one windows server 2008 R2 64bit.
Run cbworkloadgen.exe to insert 10000 items as in command below.
C:\Program Files\Couchbase\Server\bin>.\cbworkloadgen.exe -n localhost:8091 -i 10000 -r 100
Here is the output from the script when it stopped.
Thread 0 - average set time : 0.0178349999905 seconds , min : 0.0149998664856 seconds, max : 0.077999830246 seconds , operation timeouts 0
In the bucket, it shows only 9962, missing 38 items.

C:\Program Files\Couchbase\Server\bin>.\cbworkloadgen.exe -n 127.0.0.1:8091 -i 10000 -r 90 --prefix=test_
Thread 0 - average set time : 0.0184840714578 seconds , min : 0.0 seconds,##] max : 0.0620000362396 seconds , operation timeouts 0
In bucket, it shows only 8963, missing 37 items