My guess is they had multiple shots/takes and they ended up combining two of them without thinking about the continuity error.
It's such a simple thing- dump ice water over a phone- the crappy edit just diminishes it (and makes the "challenge" ring really hollow.) I already knew the phone was water-proof- a badly-edited video of it (where clearly the voiced challenge isn't actually real time) was one of the absolute worst ways of showcasing that.
I get the idea; do a parody, donate to the charity, and challenge the competitors to do the same while sneaking in a plug for the S5 being water-proof. (Which by the way, some dweeb with an iPhone and a tech-blog completely missed that point obviously, as the challenge isn't about NOT actually being physically able to dump ice water on yourself -or your product- but simply DOING it and donating to the charity, then challenging 3 others to do the same. UH-DUH.)
But the execution is just bad- there's no good reason for a fake-out edit of something so simple. Implies they couldn't get something so simple right in one take and in less than 34 minutes, which is just... lame.