> Can anybody explain to me why the curly bracket begins way below > the first row even if its definition is put in the first row?
That appears to be a peculiarity of the interaction between eqn and tbl, which looks like its aligning the top of the equation with whatever alignment point was specified in tbl. You can see this if you try top-aligned and bottom-aligned placement in tbl instead of vertical centering. For vertical centering (and if your equation has approximately equal height and depth), you can usually obtain the desired result by saying "space 0" inside the equation, which overrides the default extra pre- and post-line space requested by eqn.
