Not a scientist, but I'll try to answer.

Gale Crater is huge. It is 154km (96 miles) in diameter. The meteor that created it would have been massive, and the impact was so big that it caused the surface of Mars to rebound from the hit, causing the mountain in the middle to form. That mountain is called Mount Sharp. The Curiosity rover has been exploring this mountain, and that's where these samples were taken.

The meteor could have brought water with it, but the impact would have been so violent that it all would likely have blasted away from the crater, and any remaining would have been boiled away by the heat of the impact. That water could have flowed back to the crater eventually, through streams or rivers and possibly precipitation.

The fact that these samples are from high up the side of the mountain do bring the question of if this is a sign of water, how did it get there? I believe this is where the researchers are suggesting that Mars could have been very wet.

But as always, these are only theories. The Rovers can only do so much, and until we can get samples back, or boots on the ground, the researchers are left saying "This is probably why, but we could be wrong."