supposedly waterproof phone water damaged

Absolutely ridiculous! I have sony xperia z3 that is now liquid damaged, yet it is a waterproof phone. I closed all the covers as it is supposed to be at all times, and the worst of all is that it got water damaged by having it in my slightly wet pants! It wasn't even fully exposed to water. I had a samsung s5 and this has never happened to it before. This is very bad for sony as I thought it is better than the other brands, but obviously not! I am very disgusted as my 6000 rands phone is now useless because of something it is suppose to be resistant to!