The problem is the inconsistent result.
Ordinary crystalline white sugar, when you run it through a coffee grinder OR food processor OR a small spice grinder produces powder with various sized crystals.
Commercial powdered sugar is produced by grinding, then sieving multiple times.
* Icing sugar, confectioners' sugar, powdered sugar containing 3-5% starch has a 0.007 - 0.11mm Crystal size
* Caster/castor sugar, superfine sugar has a 0.29 - 0.34mm Crystal...