What is the purpose of this AIO filtration chamber feature? Related to that, what ridiculously dumb things have happened in your tank?

I am curious to know why the two overflow chambers don't go down to the very bottom of the tank? The diagram below shows the design of the filtration chamber and you can see that the very bottom of the tank can be accessed only within the 8.7 inches at the center. This is the chamber that the return pump goes in.

I am sure there is a very good reason for this design but I have no clue why the right and left sides have the 1/2" tall spaces under the weirs/overflows.

I have a yellow watchman goby that managed to swim through the weir on 6/22 (two days after I got it) and it has been back there since. I am pretty much at a loss for how to get it out at this point. The fish is small enough to easily fit underneath the glass that divides the overflow section from the very bottom of the tank. Even if I pulled out the return pump the fish can just back up and hide under there. There is so little room in the chamber that I can't even fit a teeny tiny custom acrylic fish trap down there because of the return pump taking up most of the room. I don't want to pull out the return pump and leave a fish trap back there in hopes of catching it. That is probably my only real option though but I'm not planning on trying at this point.

I know that the fish swam through the weir because the day I put it in the tank I saw it swimming through and hanging out on top of the foam filter pad I had in the top chamber of the custom caddy. I didn't realize it at the time but the caddy isn't a tight fit in that chamber and there is room for it to move a bit. Enough room for this fish to swim down the gap between the tank glass at the back of the display and the caddy. When I took it out of the caddy to put it back in the display, it swam right to a little cave where it stayed that night so I figured it had found a safe spot.

Since then I have basically decided to feed it and let it live back there in hopes that at some point it will be to big to fit underneath those sections and then I will have a chance to get it out. All I can do is laugh about it really because it seems so ridiculous when I think about it. The fish was small enough to fit through the weir on a 25 gallon AIO tank. Then it swam down the front of the IM custom media caddy that is in that chamber, up the next chamber where my Bullet HOB skimmer was and down into the depths it now calls home. And now the pistol shrimp I have lives inside of a rock by itself. At least I know where it is and can leave mysis shrimp at the front entrance to it's lair. Would be sweet to see the two of them hanging out together in that little cave though. Maybe some day!

Here is the escape artist and the cave that gave me a false sense of security:


I hope an explanation for why this feature exists will make me feel like this scenario isn't so ridiculous.

Anyone else have something like this happen?
I've have a brittle star living between the 2 layers one my overflow. Some tanks with full height overflow boxes have an outer "shell" with holes half way up to I guess cause water feom lower in the tank to go down to the sump. Well. The brittle star decided to climb up into the air and back down between that small 1/2" space. Been like 2 years now. No idea how to remove him.
It forces the water to stay in each chamber and go through whatever filter media you have in it.
I figured it was related to water movement through the chambers but didn't know for sure.

Would it be less effective if the divider that is 8.5 inches tall went all the way to the bottom and there was no extra chamber beneath only connected to the center chamber?
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