Got it.  <summary> isn't sorted.  It holds position with what is below it so if something gets added that sorts before that line, the summary will move down to maintain its relative position.

Since most enums just add to the end, this doesn't happen in general so I don't think it's worth spending any effort to try to fix.  When it happens, it just needs to be manually sorted a second time and then it'll stick.

That doesn't seem desirable behavior to me. Can we change the logic so it's always at the top rather than anchoring to what's below it?

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/e11e9a668e39a1ede332793ba8d677664d057f32

commit e11e9a668e39a1ede332793ba8d677664d057f32
Author: Brian White <bcwhite@chromium.org>
Date: Tue Nov 07 14:14:12 2017

Fix summary tag in HistogramNameHash.

<summary> is held in position relative to the line below it so adding
an int value less than the existing lowest will cause pretty_print to
move the summary down along with the line below it.  It then has to
be manually moved.

Bug: 781822
Change-Id: I1af804755632ccb040562e17843e2d029f0dc649
Reviewed-on: https://chromium-review.googlesource.com/755099
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#514462}
[modify] https://crrev.com/e11e9a668e39a1ede332793ba8d677664d057f32/tools/metrics/histograms/enums.xml