Defense theories of the case tend to get crystallized early, probably by the adjuster. And once the theory is in the file, you tend to "see" things that support that defense theory and overlook others that don't mesh. If the neck bruises were buried in an ER intake report, and the adjusters and defense attorneys were running on a theory of bulimia or diet abuse contributing to her condition, I could see how everybody could look right past it.
In an ideal world, of course, the trial lawyers fly-speck every piece of paper in the file. As a practical matter in a busy office, the only person who ever looked at the ER intake report was probably the paralegal who Bates-stamped it while getting ready for discovery.
I try not to attribute to malice or evil what can be attributed to incompetence or carelessness . . . Occam's Razor and all that.