When you tighten the calipers, it may move under pressure, so if it was centred, it won't be after tighten the bolt.
Also the mount may need facing to allow the calipers to sit perfectly.
The mount needs facing - I got them to sit correctly but after a few miles riding the caliper almost slips out of alignment because it's not bearing onto a flat surface... Irritating. Thanks for your help.
@Sebbb started
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When you tighten the calipers, it may move under pressure, so if it was centred, it won't be after tighten the bolt.
Also the mount may need facing to allow the calipers to sit perfectly.