Lopsided grape vines

[ameno]

I have two grape vines on my allotment I planted last year. Both are grow away nicely now, but both are exhibiting the same strange lopsided growth.
Both are train with two "arms" of old wood growing from the trunk, one to the left and one to the right.
On both vines, the new growth sprouting from the uphill, west-pointing arm is significantly more vigorous than that from the downhill, east-pointing arm.

I'm not too worried, as even the less vigorous side is growing fairly well, but I just thought it was really strange that they are both doing the same thing, and wondered if anyone might have an idea as to why it is happening?

[devonuk]

The clue is in "uphill" I think; if you have an apple espalier growing like that you raise the weaker arm closer to vertical and bring down the more vigorous, to speed up the weaker one and slow down the stronger.