It's not that the higher temperature is getting you less conversion. It changes the type of sugar being created. At higher temperatures a different enzyme is more active that creates longer chain sugars that are less fermentable by the yeast. So at a higher temperature you will generally leave more sugar and body in the finished beer. This is desireable in many styles like porters, stouts, and beers that should have a big malty backbone.
For lighter style beers you mash lower to create more sugars that are fermentable and create a drier beer. The generally accepted happy medium for an average beer is in the 152-154 range, and then brewers move up or down from that number based on how much body and residual sweetness they want in their beer. John Palmer explains this in a bit more depth in his book "How to Brew" which can be found free on line.
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