Fred
That is a great question - and I have no idea why - can you share the model with me by email and I will take a look. Normally I would put this down to
1) Shadows / shading in the view - try turning Shadows Off and see if that helps
2) The elevation of that area being slightly higher than 20.80 i.e. 20.82' yet the label still shows 20.8 (that could be it as I got a similar effect on my test model when I did that despite the color range ending at 20.8 - it was slightly darker
3)The Transparency setting - but this all looks like one surface so all is set to 20%
4) If you use a Black Background but have a PDF with White Background behind some but not all the objects, and have a Transparency set on the surface, those elements that are over the Black background will be darker than those over the White Background. The same could apply if you have a Subgrade Surface underneath one but not all of the pad areas etc. - if you set the transparency to 0 what happens?
Otherwise I got consistent results when all the elevations were in fact 20.8 and all were above the same white or black background etc.
If these don't work Fred, send me the file and I will take a look for you
Alan