I don't have an Amiko mini hd t2/c myself, but in general, if you are going to use a NIT or Network scan, then the frequency you enter must be the one that contains the Network Information Table.
In mickymicky's case (and mine), this will be 666.750. This network frequency contains the info of all the other frequencies used in that area, enabling the box to scan only the frequencies that it needs to, enabling a much quicker scan than you would get from a brute force or blind scan.
I don't know how this box works, but perhaps if it doesn't see the nework data on the frequency you entered, then it defaults to a brute force or blind scan?
It just so happens that the frequency you entered (which was actually the NETID), coincidentally happened to be a multiple of 8MHz away from the real network frequency (667 - [8 x 32] = 411). Perhaps that's why it scanned.
Most boxes will actually accept 667MHz instead of 666.75MHz if the box doesn't allow you to put in the full frequency.