Skeleton trap seemingly spawns too frequently

In a large ocean biome, I found 6 skeleton-horse traps floating in the water during a single thunderstorm. This seems to be inconsistent with the published probability.

Furthermore, it's too easy to defeat the "four horsemen" when the trap is in the water. After they spawn from the lightning strike, the horses will forcibly dismount their riders, due to the water depth. Maybe skeleton traps shouldn't spawn in/over water blocks.