I think it's more "liquid" proofing.
Lots of people use their phones around food and drink. Same people tend to spill those drinks on or around their phones.
Waterproofing would imply to those people that they don't have to worry that they spilled their latte on their phone at Starbucks, their soda on it at McDonalds, or anything like that. And yes, around the pool as some find themselves accidentally tossed in by 'friends' from time to time.
They can be careless and not suffer the consequences (or so they might think).
But labeling it "liquidproof" thereby implying that that same segement of the customer base is careless and accident prone (they are) would alienate those customers. So, the manufacturers simply call it waterproofing.
My take on it at least.