[David L.]

What I had in mind was, in what situation would a person say either of these? When I'm in a situation, I don't have to think and decide, hmm, shall I use present tense or present perfect here??? There is a situation and that situation already has meaning in it. Let me show you:

I know that the UK has strict regulations governing the safety of electrical appliances, and what can be sold as 'passed'.
I am overseas in some very small third world country, and see an electrical appliance for sale. I ask the very honest salesman whether it meets UK standards, and he says:
"These appliances are untested so far."
The meaning I gather from this is, that it is not tested and certified as meeting UK standards (so if I try to bring it back into the UK it may be rejected as 'unsafe' by customs; and if I manage to get it through, I may get electrocuted when I use it.)

I visit the building in the UK where the electricians work who test appliances being imported from overseas. One of the electricians points to an array of appliances which have all been rejected and will not be allowed as imports. He points to another table and says:

Electrician: "These appliances have not been tested so far."
The meaning I gather is, that testing is underway, and that they are in the process of testing a lot of appliances, some of which have been rejected, but haven't got round to these yet. They will be testing these later that day, next day, sometime soon to determine whether they do or do not pass.

Each of these two men chose an expression which was appropriate for the situation. That's the difference and how one decides!