Think of it as four "wheels" (or four axle points) vs four tires. With a 4x4, there are four points to potentially provide power from the axle to the road surface (regardless of how many contacts/tires there are). The same with 6x6 - there are six ends to the axles with potential for all six to transfer power.
What you're referring to as 6x6 is actually only four ends of axles - all four potentially transfering power to the ground (through six tires).
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