I faced this problem in initial my days.
At that time i had a requirement to show the check box on each row of Data grid. Here user can selects required records and do further operations.
For that i used the concept of Item renderer of data grid and placed the check box.
This works fine if there are less records than the data grid height(default grid row height = 22, so Data grid height = No of records * row height).
But if data has more recoords, at that time check box behavior is very strange.For ex : Even if i select 2 records among 20 records , move vertical scroll up & down – Its showing some time 5 records selected, sometime 6 records selected.

I observed this type of behavior to all renderers not only with check box.

For this i did small change in my data provider collection.

Here i am attaching the source code….Have a look on it.