Obama's in office and his numbers fluctuate. By this logic you could point to periods in Clinton's term that he was a horrible president. You're also citing a sole poll. What is generally found is that perception of a president softens the longer they are out of office. That views on Bush are still as poor as they are is telling of his unpopularity. I don't think you're grasping how hated Bush was towards the end of his presidency. Poll after poll showed it. Obama has nowhere near the levels of unpopularity Bush had.