The existing backsplash was tile, which had to be demolished. You don’t need concrete backer unless the tile will be in an area that regularly gets wet (shower, bath, floors).
The client selected a glass mosaic tile. The standard 15% excess over estimated of materials can go as high as 30% when you have a pattern that is not regular on all sides. If you need a ‘bottom’ of a mesh sheet of mosaic tile, that may leave much of the rest of the tile unusable, if the bottom is different than the tops or sides.
Contrast that with, say, a square or rectangular tile, and the pieces can be repurposed once one side is used (two ways for a rectangle, and four ways for a square).