the number of docs in each group depends on rows

Hi, 
We used Solr Cloud 7.1.0(3 nodes, 3 shards with 2 replicas). When we used 
group query, we found that the number of docs in each group depends on the 
rows number(group number). 

When the rows bigger then 5, the return docs are correct and stable, for the 
rest, the number of docs is smaller than the actual result. 

Could you please explain why and give me some suggestion about how to decide 
the rows number?