Simple. While it was actually a House bill, it had been amended (by 100%). So, the House had to pass it again...as amended.
“While it was actually a House bill, it had been amended (by 100%). So, the House had to pass it again...as amended.”
There were 2 House bills. One was their monstrous version of health reform (also weighing in at many hundreds of pages). This had no prospect of passage following Scott Brown’s victory. So, as you say, the Senate picked up another House bill on a completely unrelated issue, (but which had some minor tax provision in it so that it could be technically but misleadingly said to be a revenue bill originated in the House) stripped the entire contents of the bill and substituted the monstrosity that became Obamacare.
This version then went back to the House for approval and then they passed a separate “reconciliation” bill that fixed the things the House didn’t like, but because it contained reconciliation provisions, this “clean-up” bill could be passed in the Senate with only 51 votes etc. So in the end, the Dems just barely adhered to the letter of the law/Constitution even while massively violating its spirit.