Why Levels mid-point slider and Exposure Gamma Correction slider not the same?

Both the mid-point slider in Levels and the Gamma Correction slider in Exposure adjustment appear to modify Gamma.  They even use the same numeric scale.
When Gamma <= 1, both sliders appear to produce the same results (confirmed with a couple of random sample settings and using the Difference blend mode).
When Gamma > 1, however, the results are very different.  Levels produces more what one would expect in terms of moving the Gamma curve without moving the white and black points but the Exposure Gamma Correction slider seems to do something very different, either moving the black point or applying a very different curve the produces much brighter tones in the dark shadows.
Can anyone explain why and how these two adjustments produce different results?
Is this a bug with the Exposure adjustment?
By the way, in my limited tests of this I was just using a standard RGB file in 16-bit mode - nothing fancy like 32-bit or LAB or anything like that.  Also, Photoshop CS6 Extended.  Both 32- and 64-bit versions show the same behavior.
Thanks.

The differing gamma adjustment results also occur in the OS X version of Ps 13.0.1
The situation is worse than you realize, Dennis. Test with a linear grayscale gradient in 16-bit or 8-bit RGB mode instead of arbitrary images. Remember to view at 100% zoom for an accurate display.
Gamma adjustments of less than 1 performed by Levels adjustment layer and Exposure adjustment layer do differ but to a less obvious extent than when the adjustment is greater than 1. Differences can be seen in histograms, and low level differences when differencing layers can be made more obvious by adding an Exposure adjustment layer with the exposure increased.
You'll get further different results by using the gamma adjustment in the "Exposure and Gamma" method of Image > Adjustment > HDR Toning. Notice that the gamma value there has to be specified as the inverse of that in Levels and Exposure.
I haven't bothered to calculate which, if any, of the three gamma adjustment controls gives correct results because that is Adobe's responsibility.
It's quite ridiculous that a common routine giving consistent and correct results isn't being used for all these gamma adjustments. Code re-use makes software more robust, simpler to test, simpler to correct, leaner and overall easier to manage. But what do I know.

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