When guardians summon, they can continuously spawn around the area. This glitch has confirmed been in the game since 1.8 as well. At times, I have seen a guardian temple have more than 2000 guardians inside of it because of this. Here's a video, demonstrating how they keep spawning above the entity count. https://www.youtube.com/watch?v=DVUAZzOy_mI&feature=youtu.be

If the video isn't done being rendered, it will be within the next couple of hours.