trojan windows defender

I change over to windows defender a few months ago and I just had my first fake trojan.
I just go to very safe sites and got hit with the fake trojan, sorry, forgot to write any info down.
I had seen it before on friends computers and cleaned it out, it wants you to call the number but all you have to do is disconnect from net and turn off computer, turn on computer and run ccleaner and defender, running mrt now.
My question is how did it get passed defender, I know some web sites get a hidden infection and if you go there you could get infected but how did defender let it in, it may be new code in it but it is an old one, just turn off pc.
I know antivirus just plays catch up before they can write code to stop something, still I am wondering why it got through.
I am probably in the top ten percent for safe computing.

mrt has picked up 1 infection, still running

mrt said 1 infected file while running, when finished it said no malicious software found, under details none were listed, does that mean it took it out or should I run again. I might get a minor infection once every 2 years, I have had defender a few months and got one, just curious, I also read that defender will be improved in the fall update.

running mrt, it again says 1 file infected which confuses me, especially if when done it says none.

I've seen it before. It seems to only happen with Microsoft Edge. All you need to do when it pops up is CTRL-ALT-DEL and kill the Microsoft Edge process. It does not affect Firefox.

Each AV has its accuracy/inaccuracy.
It takes into consideraton:
false positives
false negatives
true positives
true negatives

https://www.av-comparatives.org/wp-c..._201710_en.pdf
AV-Comparatives Performance-Test - AV-Comparatives

Potentially unwanted programs (PUPs) are categorized as malware.
Some AV products automatically scan for them, others are manual, and other not at all.

Malwarebytes has options to scan for:
Potentially unwanted programs (PUPs)
Potentially unwanted modifications (PUMs)