Most of those 335 statistic people were probably doing “recreational drugs” at the time or possibly fondling their genitals in their “safe space”.
I went and looked at the site, and their criteria. An incident where three robbers went in, shot a store worker, and were then all shot by a good guy, would be put on the list as a "mass shooting". An incident where cops got into a shootout with a criminal, and some bystanders were hit, would qualify as a "mass shooting".
Essentially, ANY incident where four or more people were injured or killed by gunfire, was counted as a mass shooting.
While all are crimes some are terror related and some are simply crimes. A few gang bangers shooting one another in a park is not the same as San Bernadino.