For me it is also a matter of the fabric quality I use. Some fabrics are very easy to needle, with others (with a lower thread count) it's not so easy. On some of my quilts the stitches on the back are even and consistent and on others they are not, although I used the same batting and the same thread brand.
I have to confess I don't care too much about the back of my quilts. I always try to do my very best - that is all I can do! :)