This seems to be one of those random happenings that you may never figure out. My favorite so far is the "Dicky Bird" theory, your bullet has encountered something along the way. Although bullet manufacturing processes seem the be quite perfected, there is always a chance of an anomaly, maybe that bullet has an inclusion in the core, throwing it off balance, it's not outside the realms of possibility.
Bit of crap in barrel channel?
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