They're mixing up their metric and imperial sizes. These are from Forgefix, which are a British company. Many British people aged about 55 and over tend to still use imperial sizes for some things.

So it's not 6-8mm, but "the size of wall plug for 6-8 gauge screws, which are 3.5mm - 4mm screws in metric.

Strangely, if you buy a multipack of masonry bits, they give you 3, 4, 6, 8, 10mm bits. Some brands of wall plugs tend to use 5.5mm, 6.5mm, 7mm. It is a conspiracy!

Your options are:

Use a smaller drill bit and wiggle it, just a little bit
Use a larger drill bit and pack it out with a shim/matchstick/chopped off half-a-wallplug
Individually buy the masonry bits which correspond to the plugs
Buy a different brand of wall plug, which uses 6mm or 8mm bits