Item stats are formatted as stat.statType.namespace.item_name. For example: stat.useItem.minecraft.fish, or stat.breakItem.minecraft.wooden_sword.

Conversely, entity stats are formatted as stat.statType.EntityName. For example: stat.killEntity.PolarBear, or stat.entityKilledBy.SkeletonHorse. These names are inconsistent with the actual entity names, which are now minecraft:polar_bear and minecraft:skeleton_horse in the above examples, and generally follow the namespace:underscore_separated convention.

Essentially, the entity names used in entity stats are wrong, either using the old entity names, or new entity names formatted similarly to pre-1.11 convention. In addition, they lack the proper namespace.