Ranking at siege is decided by the amount of AP you gain WITHIN the contribute zones (both you and the target MUST be inside, otherwise it doesnt count)
This calc is done individually and that means if you are in a group or in an alliance every AP gained from a player or object killed will be shared with all the group/alliance member in range.
To make an example, if you kill a player which give you 1k AP as solo but you're in an alliance of 10 ppl you will gain 100 AP from it, and thats your personal contribution which will be comparated with all other player of the same class youre playing which made contribution aswell (ask gunners how much competition they have ).
Also consider this: an npc/kill/boss count 10x times a PvP kill (on our server 99% of npc doesnt give AP anymore btw, only special npc of opposite faction which spawn sometimes and balaurs from dredgion spawns counts atm, in short: it gives you AP and its in contrib zone? it counts.) which means if you're attacking a fortress you want to get the higgest amount from the gate and from the boss, to make another example: if you kill the gate and get 400 ap you get the same contribution amount you would get from a PvP kill which gave you 4k AP.
And there you would think: ok so I still can do PvP only and get rank 1. No. Our AP limit system with a ridiculous limit will ruin your entire siege.
On defense fortress you want to get the best amount on PvP since (beside dredgion and special npcs I mentioned before) theres no object to kill to gain contribution, and also there you have to balance the amount of AP you gain from every fortress to not being in the situation where you did A LOT of contribution in a fortress but your AP gain is locked so you cant do contribution in other ones (this is typically the case of MKK siege)
And thats it.
Also: ranking spots are limited and that means you may have done a lot of AP and still get a lower rank you expected because many other players of your class did more than you, reason why ppl is running AP boosts at siege.
If you are asking, yes. This system is bad af for a mmorpg since forces you to go solo or duo maximum to get max ranking instead of promote group playing.