dataFetchSize functional limitation

Hi Team,

In List grid we have a property called dataFetchSize,
wen we set the data fetch size as 30 upon scrolling for more records we can see 30 at a time only.

Means first time when 30 records were loaded, up on scrolling next 30 records loads. What abt first 30 records will they be removed from the grid scope or grid still contains in another object.
Is there a limitation of Grid to show only 30 if set dataFetchSize as 30.

Please advice.

I m using latest prod build 9.0 version.