I've confirmed that all my rows will always have the same size. The issue here is about the borders / line-space / padding that gets put into every row through CSS.
In the non-data ware lists the height these properties take will be the amount of height that falls away under the grid when data-aware = False.
But when set to true but still use manual load data (so its the exact same data as before), it will take the CSS bordering e.c.t. into concideration