Help me understand what I'm missing?

Spring pocket depth can vary quite a bit on factory heads. If the stem heights look reasonably even with a straight edge, your measurement from valve tip to spring seat .020-.030 variance may not be out of the question. Using a longer pushrod would take up the slack. Another fix would be using lash caps which are normally found in .060 or .080 thicknesses. Preference would always be to have lifter preloads fairly even, but as long as the lifter is preloaded and not bottomed out, I wouldn't worry about it (at least not worth pulling the heads to correct).
It may be a good idea to pull the lifter on #6 exhaust to take a look since it is so much looser than the rest.