Easy explanation. You see, Israel isn't the be-all/end-all issue for most American Jews. (Nor should it be). Liberal Jews are pro-"choice," anti-gun, pro-welfare state, pro-affirmative action, etc. .....you get the picture? With a resume like that, why wouldn't they support liberals? They are liberals.
Now on the other side of the coin there's people like me -- (politically) conservative Jews who are pro-2nd Amendment, anti-tax, anti-big gov't, etc. Israel, while certainly an important consideration, is far from the top of the list.
Seems a lot of non-Jews believe that Israel tops the list of importance for Jews on both sides of the political spectrum, and it's simply not true. Certainly for some it's true, but they're in the distinct minority.
You're right...I got on a list of old rich Jewish liberals sending me very pro-Israel stuff. The other day I forwarded this list Dick Morris's column asking how Jews who support Israel can vote Democrat today...that the Democrat party is now the home of the new anti-semitism. I can't tell you the responses I got back. to paraphrase.."I WILL NEVER VOTE FOR A REPUBLICAN...GEORGE BUSH SHOULD DIE...I CARE ABOUT ISRAEL, BUT I CARE ABOUT AMERICA TOO...