Main Gear on 777 and A340-600

[luvmuhud]

Can anyone shed some light on the following questions:

What is the theory behind the A340-600's main gear design (left and right mains are standard bogey trail, with an opposite arrangement for the centre main)?

Why did Boeing opt for the 6 wheel arrangement on the 777 vice a centre gear similar to the A340-600?

Thanks in advance.