This is my big theory on everything which also explains that.
[spoiler]The island was covered by a dome of sorts, by the big electromagnet. That's why nobody could ever leave the island, you try and you just get sucked back in. That explains why Michael and Sawyer got sucked back into the island even though the current really wouldn't have done that. It shows how Desmond just ended up circumnavigating the island once he found it, and was unable to escape, even with a boat. So when the hatch blew up, the electromagnet was gone, the dome uncovered and you could see the other island. That's why the others showed it to Sawyer. They knew he would try and escape, see the island and be all wtf. So they showed it to him so that when he sees it, he sees it as I'm screwed instead of as freedom. Now that the electromagnet is gone, they can escape if they tried which is why the others were so frantic to get the boat. It really makes no sense, based on everything that we've seen, for them to be on another island.
And just for fun, another theory.
There's another group of others on the island. The others we've seen really don't follow the whole walking in the jungle barefoot thing we saw in season 2. The other group of others are much more savage, outcasts from the original group or something. That's why the others always say they're protecting the island crashes, and how they aren't the enemies. So later on in this season, we'll encounter the other group of others and shit will commence going down.[/spoiler]
