Photoshop :: Levels Preview On Channel Is Different To Result?

In CS3 I use a Dust & Scratches filter on a layer in a high resolution (3200+ppi) 16bpc image, then traditionally change the layer blend mode to Difference, make a duplicate of a channel which has the clearest information of where the filtered changes were made, and use levels to increase the contrast on a copy of that channel. Occasionally I get this result though - the Levels adjustment preview shows spots clearly, but clicking OK results in something very different from the preview. This only seems to happen occasionally.and result looks like this:

As the purpose of the channel is to make a mask, the result isn't as good as I'd like. I assumed that the problem might have been the grey starting colour of the channel, so I pushed the 0 up to 9 in an attempt to get a uniform K value of 100% on the darkest areas, and then ran a second Levels adjustment to make the white spots brighter. The second levels adjustment resulted in pretty much the same result as above, again, not what was shown in the preview. I also assumed that it may be down to numbers and that levels is occasionally too limited to deal with all the shades of grey on a 16 bit channel. Probably wrongly as this normally works exactly as required.
 
This only happens occasionally, and isn't that big a deal - I can quite happily go and use a Curve instead. I'm just wondering why the preview in the adjustment window is different from the actual result. The fact that it only happens on some of my images is what concerns me - purging all beforehand does not make any difference.how to get accurate results in the preview? Is the preview using a different method to show the results? Unfortunately I won't have a copy of CS6 to try this on until a couple of weeks from now, so not sure the best way for someone to try and replicate it on newer versions.