Because it's quick and dirty. Here are OPS splits from batting position for last year across MLB.

4th 805
5th 779
6th 748
7th 735
8th 707

And as a group the 4-8 spots posted a 756 OPS

I suppose it depends where guys end up slotted there but I can see some of our hitters outdoing that. As a nervous Swanson believer I feel he will be better than average in the 7th or 8th spots. I feel Acuna would likely be average in the 4 hole but above average everywhere else.