Taking abortion for instance (not something I would have brought up but since it's on the table). 28% of people think it should be legal in all circumstances 21% think that it should be illegal in all. Then you have half of the country's population in between who think it should be legal only under certain circumstances. I'd love to see the breakdown on "certain circumstances" but if you look at it in the number s middle of the road on the issue is actually supporting certain reasonable restrictions.
I've not looked lately but the only Dem Senators I remember having a less than 100% NARAL rating were Pryor (gone) Landrieu (gone) and Casey.
There are at least a couple problems with your example.
First, as I understand them, NARAL's ratings are based on how the candidates voted on issues that were up for debate in a given congressional term, not their general positions on a few issues of utmost importance to NARAL. In other words, it is not grading whether the senators support abortion in every single instance. To me, the fact that every Democratic senator voting to confirm Elena Kagan and for expanded birth control coverage doesn't suggest that they are so far radically to the left as to only be in sync with 21% of Americans.
The NARAL thing would only be interesting if everything that came up for a vote was radically left or radically right. But most bills are just things that tinker with marginal regulations and don't really move the ball in either direction. What was the last issue relating to reproductive rights to come out of Congress that Democrats broadly supported, but seems very far-left and out of step with main stream America? I am struggling to think of a good one - IIRC, even the 2003 ban had some Democratic support.
Second, abortion is the bread and butter of both parties' base, so to win primaries, candidates have to take far left or right positions on that issue. That will probably always be the case. Gun control, environmental issues, economic issues, etc - the is more variation. The rest of it, you will probably see more nuance.
I do agree in general that the country is becoming more polarized. But I don't think the Democrats are moving farther to the left. I think the Republicans have shifted much farther to the right, which has forced Democrats to be more solidly unified in their opposition. As the Republican Party moves further right, people are jumping ship and Democrats are moving to the right to try to pick them up.
Re-rankings. Abortion isn't something I really think or care about so it's not something I generally stalk info on. The one and only time I've looked at NARAL's ratings was in a year containing several random abortion related bills and only because there were so many that it was in the news a lot. I want to say it was 2009? Maybe? I can dig around a bit after the person currently throwing my pans in the trash can goes to bed.
And I agree with you on the second point. It's not an issue I even think of when placing people on the political spectrum.