I don't understand Smogon's tier judgment

[Alfieri]

One thing that always confused me was how Smogon decides the tiers for their Pokemon. For example, let's take Manectric.

Manectric by itself is considered RU in the previous generation, and coming into X and Y, it somehow became more useful thanks to its mega evolution and thus became OU. However, how can Manectric be considered OU when it is Mega Manectric that is OU? Shouldn't Mega Manectric be OU and leave Manectric to be RU? Why does it affect the Pokemon in its entirety?

In contrast, someone like Salamence or Gengar. Salamence is UU and Gengar is OU on their own, yet their mega evolutions are classed under Uber. So how come those Pokemon don't get the same treatment as Manectric to jump to Ubers along with their mega forms?

Is it because Manectric is actually useful on its own in an OU battle and I just completely missed it?

 

[Nah]

Gengar's and Salamence's Megas are in Ubers while the base forms are in other tiers is because they were deemed broken in OU and the community banned them from OU. Ubers is the only tier that's not usage based.

Mega Manectric on the other hand is not broken in OU. Both the regular form and the mega form are OU because while people always mega evolve it asap you still have to bring the base form into battle. So the regular form is technically used enough to be OU.

 

[Polar Spectrum]

^ Basically.

Mega Gengar and Salamence are Ubers while their base forms are OU / whatever because you're still allowed to use the base forms elsewhere, their viability and popularity placing them in those respective tears. Mega Manectric and Manectric are both OU, because Manectric is by popularity as Mega Manectric, in OU. I would assume, corresponding to your question - that Manectric would drop tears if its mega were suddenly decided to be broken, where it would return to whatever lower tear it was in before X/Y, and the mega would bounce up to ubers. Also in relation to your question, I would assume Manectric alone could be used in RU or something - but Manectite would be banned in it. (a la pinsir / heracross and their megas.)

Correct any of this if it's inaccurate; because I do not pay the closest attention to these tear rankings

 

[Zeffy]

The tiers are based on usage (which should be pretty obvious, given that most of them has the suffix -used). Basically, the regular forme of the Pokemon is the basis of a mega evolution's usage since it is the form you initially bring into battle.