when they are put on the discs are loose, they tighten with the wheel nuts, if you look closely you will see the holes are threaded, this is because after years of them being on there and not moving, they rust in the gaps and become really tight, basically they are there for you to screw a bolt into when replacing them, you wind the bolts in evenly to to pust the disc off, the positioning of the holes are just different depending who made them.
so as long as the discs are tight when the wheels are fitted and tight, then all is good.
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