In a four-stroke engine, any cylinder fires once each 720* of crankshaft revolution. An 8 cyl will therefore fire a cylinder once every 720/8 or 90* of crankshaft revolution.
A 6 cyl should fire a cylinder once every 720/6 or 120*. HOWEVER - a 90* V6 cannot do this unless you make the engine longer by having a separate throw for each cyl. (Draw it up, you'll see) A 60* V6 has no problem with a smooth, even-firing order and lighting one off every 120* (and usually has a firing order like 1-2-3-4-5-6)
Because of the crank design on the original (V8 derived) 90* V6 it could NOT fire cylinders with an even spacing of firing order (oddfire)
Later versions DID use separate throws for each cyl to get the spacing right (evenfire) but this necessitated a whole new block with the banks more staggered than is the case on a V8