question about online chart (must..plus)/tiresizes

I've got a question about the chart/image I've seen several times.
It was listed at
http://home.earthlink.net/~myradpc/_uimages/turn52.JPG

and says its from mustangs plus (originally that is).

Anyway, the tire size for the 15X7 and the 15X8 are listed as the same tire size. I guess I'm curious why these are listed as the same tire size? (225 60 R15 btw)

The reason it matters to me is that I just ordered some wheels from summit.

Cragar, 2 15X7's and 2 15X8's. The 15x7's have 4 (and) 1/8" backspacing, the 15X8's have 4 (and) 1/4" backspacing.

Of course tires are next, so I'm a little confused. Any light on the subject?

I've seen posts of these sizes on the wheel sizes I have.

(btw, F = 15X7 and R = 15X8)
Setup #1
(F) 225-60x15 (R) 245-60x15
Setup#2
(F) 205-60x15 (R) 225-60x15

Which compensate for the 15x7 and 15X8 difference. So that confuses me more about the mustangsplus image.

Thanks for any advice.

..