Sometimes it's also a matter of convenience.
Earlier this week I had to run a sub-feeder and the required cable size calculated out as 16 sq. mm. That's not a size that's used very often here and I had none available (and a 50-mile round trip to get some). So I used paralleled 10 sq. mm cable instead.
Easier to work due to the smaller size, and reduced voltage drop into the bargain.
