At the line 55:
status = UnitThreatSituation(unit)

Was it supposed to be "status = UnitThreatSituation('player', unit)" ?

Without specifying the 2nd unit to compare against, the function returns whichever mob the 1st unit has the highest threat against, and I believe that is the intended behavior we want with it.

If you really feel the need for specifying the target unit to match the threat against, you can use the .feedbackUnit attribute.

What @p3lim said w/ minor correction, there's an attribute that does what you want.

W/ 7.0.0 I added .feedbackUnit attribute. I recommend to use element.feedbackUnit = "player" for target, focus and boss* frames in your layout code.

As for focustarget and targettarget frames, it's a bit trickier :p For instance, if you're a tank and you're watching your other tank via focus frame, you can set focustarget's feedbackUnit to "focus", this way you'll be able to see your other tank's threat against his target o_O

Thank you guys.