Your mistake was giving them an answer instead of asking how the scale was setup before giving them a number. Psychologically, by answering first your established that the question was valid as presented and it anchored their expectations as the ones you had to live up to. By questioning it you get to anchor your response to a different point.

Sometimes questions like this can be used to see how effective a person will be in certain lead roles. Recognizing, explaining and disambiguating the trap question is a valuable lead skill in some roles. Not all mind you... And maybe not ones most people would want.

But most likely you dodged a bullet.

Did your interviewer profess to be a 10 in .net, otherwise how would they know what that looks like? I was told that I'm unsuitable as a programmer of PLC because I never used their software before. That I write the algorithms that go into a PLC was not sufficient. These people are looking for unicorns but find donkeys everywhere they look.

As a hiring manager, I can understand why you didn’t get the job. I agree that it’s not a “good” question, sure, but when you’re hiring for a job where the demand is high because a lot is on the line, the last thing you’re going to do is hire someone who says their skills are “6.5/10” after almost a decade of experience. They wanted to hear how confident you were in your ability to solve problems with .NET. They didn’t want to hear “aCtUaLlY, nO oNe Is PeRfEcT.” They likely hired the person who said “gee, I feel like my skills are 10/10 after all these years of experience of problem solving. So far there hasn’t been a problem I couldn’t solve with .NET!” That gives the hiring manager way more confidence than something along the lines of “6.5/10 after almost a decade, but hire me because no one is perfect.” (I am over simplifying what you said, because this is potentially how they remembered you.)

Unfortunately, interviews for developer jobs can be a bit of a crap shoot.