Some wheels have different disk types, for brake clearance. Not all wheels have good brake clearance, so they need thicker back pads to add space. This of course changes the offset.
So for instance, you may notice on Work Wheels USA's website that they have certain wheels that have similar specs but the lip sizing is different. This is due to the change in back pad thickness. One easy way to see the difference in thickness would be to compare the same diameter, width, and lip size.
You'll notice that 19x10.5 +45 (A-disk or regular), 19x10.5 +55 (O-disk or low-disk), and 19x10.5 +28 (R-disk or high-disk) all have 72mm lips. The back pad has 10mm more metal shaved off for the O-disk version compared to the A-disk version, and the R-disk has 17mm material
added in comparison to the A-disk version. If you have the clearance, O-disk allows you to simply run bigger lip sizes, by compensating for the difference in backpad thickness (bigger lip = smaller inner barrel for the same width wheels).
Just for a reference, my new wheels were once high-disk, and were converted to low-disk by machining metal off of the back pad and filling in the lug area.
