Hi,

I tested behavior with a datalist web part on a default Corporate site (hotfix 33), page
~/examples/web-parts/listings-and-viewers/documents/datalist.aspx

The page size was really 30. I created some additional documents to test behavior. Is not it possible that some items are not displayed by transformation in your case?

For example if you insert following code to the transformation, does it return 13 or 30 for the last item?
<%# ((DataItemIndex + 1)) %>

Or have you defined all class names of documents you are trying to display in Document types property?

If you disable paging, can you see all results?

Best regards,
Ivana Tomanickova