I'm a bit confused by something here. I admit that I do lack in my knowledge of the US political process in these matters and I'd appreciate it if someone could clarify this for me.
$1:
U.S. President Barack Obama has vetoed a bill that would have approved construction of the Keystone XL oil pipeline.
The move, while expected, is still significant because it is only the third time that the current U.S. president has opted to shoot down a bill passed by Congress.
Then...
$1:
A presidential veto can be overridden if a bill gets two-thirds majority in both chambers of Congress — the Senate and the House of Representatives.
If Congress already passed the bill, which Obama vetoed, why would they need to vote on it again as the 2nd quote suggests to override the veto? I would think that in order for it to pass to begin with that the majority of those in Congress wanted it to be so.

Is it because the 2nd round of voting is specifically to override the presidential veto rather than voting on the original issue?