It is due to different variations and height differences.
Variations:
There are around 48 different DC barrel types, therefore, each one can not be provided so you may need to contact your device’s manufacturer to confirm the specifications and purchase the correct adapter.
With 3 DC barrel fittings: 5.5×2.1mm (adapters for 5.5×2.5mm and 3.5×1.35mm incl) the one built into the DC iPurifier 2 is the most common one and along with the two sets of pigtails, covers the majority of devices.
Height different:
Laptops/routers/streamers vary in height – so physically, the DC iPurifier 2 may/may not mechanically sit at the power port of the source. It may be worth turning the unit on its side so the height is reduced since the distances from the side to the centre is shorter than the height from bottom to centre by a few MM.
