Where did you get your numbers from?
"Anyhow, to pick an example of the 767-300, DL fits 254 people in (24 F and 230 economy)"
Delta has 206 or 212 passengers in their 767-300ER.
In a dense 2 class config. you will have about 244 paxe.
In the A330-200 the std. config with two classes is 293 seats, 30B at 40in + 263Y at 32in pitch.
Some Airlines have two class concepts with only ~260 paxe on their A330-200, some airlines (like canada3000) have 340 Y seats on their A330-200.
So you have definitely more seat capacity on the A330-200 then on the 767-300.
The A330-200 is as you know a much bigger and heavier aircraft as the 767-300.
The A330-200 has a mtow of 233t, the 767-300 of 186,8t.
So the trip costs of an A330-200 will off course be higher then that of a 767-300ER. That´s clear.
But the A330-200 has more capacity and therefore the slightly higher trip costs are Ok for the airlines.
Also the freight capacity of the A330-200 is higher then that of the 767-300ER.
The total cargo hold volume of the A330-200 is 4752 ft3 compared to 3,770 ft3 of the 767-300.
Also the cargo weight capacity is much higher and the A330-200 is able to carry std. freight pallets & containers, the 767-300 is not.
Max. range of the A330-200 with max. passangers is 6650 nm (253 passenger and baggage) compared to 6,115nm of the 767-300.
But normally the max. Pax. range is not used, because
a normal flight will carry additional freight to the baggage and therefore the range decreases.
Quite simply, the A330 offers a more cabin capacity, better payload carrying capability, better range and take-off performance then the 767-300ER.
And that justifies the higher tripcosts against the 767-300.
The A330 is just a bigger plane and therfore costs and provides more.
It´s the choice of the airline if the particular route fits the A330-200 or not.
The 767-400 is more compareable to the A330-200.
It´s very close in passenger capacity. It´s not that close regarding cargo, due to the fuselage geometry of the 767. And it comes not close to the A330-200 in range. But the A330-200 has higher trip costs as the 767-400, therefore the 767-400 is very costeffective on a per seat basis. It´s the real A330-200 competitor.
It really depends on the airline wether a 767-400 or A330-200 is better for their network. Things like comunality, freight capacity etc. are counting here, because the passenger capacity of the a330-200 and the 767-400 is very very close.