The tyre profile is not necessarily different because it is a different width/height ratio; the sidewall may be just bigger or smaller. You generally won't find anyone racing on 60's front tyres as it makes the tyre to stiff (60's fronts probably came about as part of the fashion fad for low profile tyres). The 70's ratio allows a little more flex in the tyre when it hits bumps, soaking them up better than a 60's and helping the bike hold a better line.
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