numFound is changing when start parameter changed

hi
I use solr4.4 , 2 shards and 2 replicas and I found a problem on solrCloud search.

If I perform a query with start=0 and rows=10 and say fq=ownerId:123 , I get numFound=225.
If I simply change the start param to start=6, I get numFound=223.
and i change the start param to start=10 , i get numFound=219.

i searhed google and get "You probably have duplicates (docs on different shards with the same id)."

how is that happen? is that a bug? and how to fix it?
Thanks

-Conan