Nothing to worry about, as Sarah mentions these rings (in general accompanied by slight changes in color) are from the speed changes during burning.
In case you don't know, many medias are not written at constant speed in all burners, and each speed change during the burn induces a difference in the way the dye is written, thus the color changes.
"It was a XBox burn, so the scan didn't really get past halfway"
Now THIS is not normal. As a matter of fact, G05 discs do not appreciate 4x burning. You'll probably get better results by selecting @8x with these. Even so, G05 varies a lot in quality and if your batch is of marginal quality you'll probably have marginal or even bad burns on your 3500.
I don't think Optodisc is a very reliable brand, if you really want to stick with Ritek G05 youd' be safer with Traxdata or Maxell.