Sometimes when I enter the portal with 5 energy, the portal only takes 4 energy and I'm stuck with 1 energy instead of 0 when I'm exploring the island. This happens to all of the pets that I explore with, as long as they enter the portal with 5 energy.

I believe this was a modification made to account for the fact that you didn't have a chance to use bandages if you had 0 energy when exploring an island.  So now you retain one energy if you enter the portal with only 5. 

Here's Angel explaining it: http://beastkeeper.com/thread/2167

Ok that's weird. Thanks though. :3