Defense always rebuilding

#1
I'm very new to the game, so I'm hoping I'm just missing something.

I know the defenses have a 70% chance to rebuild, but is there a special addition to that where the last one always rebuild?

There is an inactive near me with quite a pile of resources (well for my noob eyes they look like a lot), with 1 in every defense type. I've attacked the planet a few dozen times but all of the defenses build back every time.

8 defenses with a 30% chance of getting blown up, times 30 attacks or so... at least one should have gone poof by now.

Note: I understand I could just be at the bad end of the random number generator, but I just want to make sure.