ITA Matrix has issues with married segment logic when using multi-city searches. It incorrectly looks at individual flight fare buckets, rather than the married buckets when quoting fares. If you check on ExpertFlyer, you will see there is only D inventory only July 4th and no Z when the flights are married together. Google uses the same code, but does a final check on inventory at the end and discovers that there is no Z available. Google is wrong to suggest that an agent can book this. Agents can't override the married segment bucket inventory set by the airline.

DL1129 J9 C9 D7 I0 Z0 W9 Y9 B9 M9 H9 Q9 K9 L9 U9 T9 X4 V0 E9
DL337 J9 C9 D6 I0 Z0 W9 Y9 B9 M9 H9 Q9 K9 L9 U9 T9 X9 V0 E9

There have been several threads here on this issue.