I've not noticed this when using the plugin with CS5 (on a Mac), but now you mention it...
... a quick check suggests that the numbers are closer when I have prophoto as a PS working space and have it selected in ACR, than when all is set to sRGB. When doing my night time images for B&W I always tend to work in ProPhoto before B&W conversion
It's a small effect (within the bounds of seeing what I'm looking for ;-) ) but suggests that it may be colour management related. I've not got the chance to test this accurately for a bit, but have you tried looking at the effect you see with different combinations of image and and working space (that's if you're using PS - I don't have aperture or lightroom)