It might be counting all the expected failures whenever there are unexpected failures too. 

But this is inconsistent with what Milo shows as "Unexpected Failures".

I think when analyzer/step/test_step.go changed to use the test-results/model structs for parsing test results, we stopped checking "is_unexpected" (reasons escape me) and just check for PASS.  This almost surely is a bug. 

The following revision refers to this bug:
  https://chromium.googlesource.com/infra/infra/+/c5d09780c744070a529662b9129a4cceb5113457

commit c5d09780c744070a529662b9129a4cceb5113457
Author: Sean McCullough <seanmccullough@chromium.org>
Date: Fri Mar 09 17:35:26 2018

[som] Fix unexpected test failure identification

Bug: 819853
Change-Id: Icdc5ac8e36acf2ebc256efe93bbe177b1ef3ef6e
Reviewed-on: https://chromium-review.googlesource.com/956967
Commit-Queue: Sean McCullough <seanmccullough@chromium.org>
Reviewed-by: Shuotao Gao <stgao@chromium.org>

[modify] https://crrev.com/c5d09780c744070a529662b9129a4cceb5113457/go/src/infra/appengine/sheriff-o-matic/som/analyzer/step/step.infra_testing
[modify] https://crrev.com/c5d09780c744070a529662b9129a4cceb5113457/go/src/infra/appengine/sheriff-o-matic/som/analyzer/step/test_step_test.go
[modify] https://crrev.com/c5d09780c744070a529662b9129a4cceb5113457/go/src/infra/appengine/sheriff-o-matic/som/analyzer/step/test_step.go