Watch CSpan to see how voting on controversial issues is done. The basic idea of both parties is to minimize the need for a vulnerable legislator to have to cast a vote that might hurt their reelection chances. Thus, votes are recorded with a number of congresspeople delaying till near the end to see if their votes are needed.
Senator Feinstein is very popular, is unlikely to lose her seat, won't have to run for reelection until 2010, thus she is free to vote for needed legislation that might not be popular with her constituents, sparing another Senator from taking a position that might jeopardize his/her chances at home.