There were few enough crashes that it seemed like sampling was having some effect, but the numbers were still suspiciously high.

I've found one repro for this. If chrome is running in the OutOfReportingSample state, then it's launched again with a different user data dir, that would put it in the in-sample state, the original chrome will report a crash with OutOfReportiSample. This is because crashpad uses the default user data dir to store the crash database, and will thus be shared between instances of chrome.

Based on the crash data, this doesn't seem very likely though. There is not many crashes reporting with the --user-data-dir flag, in either the Out or In experiments.

One bug that mentions the crashpad issue:  crbug.com/565446 
And a CL comment that also mentions it: https://codereview.chromium.org/1481703002/#msg31

A more likely cause has been found. The installer shares the crash database with chrome, and the installer crash client doesn't know about sampling. When the installer runs to install an update, it will turn on crash reporting.

I've repro'd locally by leaving a canary with that was sampled out running over the weekend. Installer was run at some point for new versions of carry. Checked on monday, and that client can report crashes, and reports the sampled out experiment.

I'm going to update the installer's crash client to check sampling.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/7d2b5433e0e583fc5edb40f16abc9566ade06f14

commit 7d2b5433e0e583fc5edb40f16abc9566ade06f14
Author: jwd <jwd@chromium.org>
Date: Wed Oct 26 16:06:06 2016

Adding sampling support to installer crash reporter client.

BUG=654771

Review-Url: https://codereview.chromium.org/2422103004
Cr-Commit-Position: refs/heads/master@{#427702}

[modify] https://crrev.com/7d2b5433e0e583fc5edb40f16abc9566ade06f14/chrome/install_static/install_util.cc
[modify] https://crrev.com/7d2b5433e0e583fc5edb40f16abc9566ade06f14/chrome/install_static/install_util.h
[modify] https://crrev.com/7d2b5433e0e583fc5edb40f16abc9566ade06f14/chrome/installer/setup/BUILD.gn
[modify] https://crrev.com/7d2b5433e0e583fc5edb40f16abc9566ade06f14/chrome/installer/setup/DEPS
[modify] https://crrev.com/7d2b5433e0e583fc5edb40f16abc9566ade06f14/chrome/installer/setup/installer_crash_reporter_client.cc
[modify] https://crrev.com/7d2b5433e0e583fc5edb40f16abc9566ade06f14/chrome/installer/setup/installer_crash_reporter_client.h

You started fixing this bug over two years ago. Are you still working on it?