Why are 1/2" corcrete backer boards less than 1/2"?


I'm about to put some granite tiles down on my bathroom floor but it never occurred to me to look at the concrete backer boards more closely. I purchased 1/2" and it didn't look right so I took my tape and measured it somewhere between 1/4" and 1/2".
I know with wood, a 2x4 is less because it was planed down at the factory to the smaller finished dimension. But what's the excuse for concrete backer boards?
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Jack writes:

I believe plywood comes in off-fraction inch thicknesses, because it is actually manufactured to even millimeter thicknesses. Perhaps that is the case with this product, too.
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Richard J Kinch wrote:

To allow some room for a layer of thin-set tile mortar of course. <G>
Jeff
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I don't know what brand you bought.
Wonderboard by Custom is 1/4 " and 1/2". Usually stocked as 3x5. Durock by USG comes in 5/16, 1/2, 5/8. Usually stocked as 4x8.
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It was 1/2" HardiBacker.
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