Thanks for asking about this, Tom--I've wondered the same thing, as the numbers aren't always indicative of the climbs' comparative difficulty. Granted, I could be over-sensitive to these differences, being a fledgling trad leader and not wanting inadvertently to get onto something out of my tender range, but I have found it helpful to be aware of the context of the rating.
In Leavenworth, for example, I've noticed that many of the newer lines seem to be rated softer as compared to the ratings of "classics." (I don't know the date of the FA of R&D which is rated 5.6 as compared to Midway which is 5.5, but I find it a good example of this disparity.)
Routes which have become polished could certainly attribute to this perception, but I also suspected it had something to do with the style of the climbing found on those older lines. Routes requiring burlier moves like knee bars, chicken wings, or plastering your asscheeks against a greasy chimney wall can feel more daunting than ones which let you get away with less "old-school" technique.