I can explain. And firstly, I agree that live, DOD is an incredible song, and it slides up the listing when you consider live tracks. However, Paschendale is better for several reasons.
1) It actually happened. I had ancestors there, and the battle itself is a great piece of human tragedy, second only to Stalingrad in terms of "most miserable battles fought by mankind" and first in the annals of "pointless death". For what it's worth, Third Ypres is a very well-known part of British history (epitomized, for instance, in pop culture by the comic series Blackadder Goes Forth, which has a tragic, terrible end), whereas DOD is a story. A good story, but just that.
2) The music, being H-written, is a heavy, hard-hitting piece of music that tells the story on its own. DOD relies on Bruce to tell you what's happening, but there's no way I could hear anything else from Paschendale. The way Nick comes crashing down on the drums after the "daNAdaNAdaNAdaNAAAA", the way the guitar (H?) sounds after the "Rush of blood and over we gooooooo" part. Even the solos are great, frantic, desperate, like that of someone grasping wildly for life in a plain of death.
3) Every single member of the band is at their absolute best. I'm not sure if the same is true of DOD. I'm not saying anyone is bad, or even not great, but when I hear Paschendale, I hear perfection from every channel; I hear incredible effort and emotion in every strum, word, and beat. Again, this isn't to the lesser of DOD, which is just awesome, but I'm saying that the band put something special together - and they knew it. Look at what Davey said about why they stopped playing it on the current tour: they made too many errors and didn't want to ruin the song. Everyone knows they mess up pretty much every song every night in some way, and sometimes pretty heavily, but Paschendale, they didn't want to bollock it. I think that says something about how they feel.