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I know that block 912721 was in both UpdateTip events the same, clearly visible by the same hash. I was just surprised why it would need a second UpdateTip when this block 912721 wasn't realy affected by the fork of block 912722. It was block 912723 which decided what branch of the former tip of 912722 should be followed as the "true" blockchain (most accumulated hash work).
It updates one block at a time. To go from old 912722 to new 912723 it has to count down making 912721 the new best for only a fraction of a second before continuing with the new 912722 and 912723.