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This is not true in the way you think, it is more of a marketing statement.
The Pioneer's supposed 48x High-speed copying is calculated as the maximum attained video speed when you burn an EP encoded 6-hour video sequence to DVD. Since the normal 1x DVD writing speed is based on a 1-hour video sequence, this means that the writer's 8x speed is multiplied by a factor of 6 - i.e. "48x" in marketing speak.
The real maximum writing speed of the Pioneer 720 is 8x which means that it is optimized for up to 8x media. The newer 16x media is probably not known by the writer's firmware, and the writer will then burn it at a default or "safe" speed, which in your case seems to be 4x (calculated from the 14min + lead-out you mentioned).