I do not think this word means what you think it means.

How is it that when someone receives recognition, honor, and/or praise, or perhaps a political endorsement from an admirable person, they are always saying "I'm humbled and honored?"

It is, by definition, impossible to be both "humbled" and "honored" by the same event.

One is humbled not by endorsement, but by criticism.  One is humbled not by winning, but by losing.  One is humbled not by being told how effing great you are, but by how greatly you've effed up.  It comes from the same root as "humility" and  "humiliated" for a reason.

I think perhaps that when someone speaks this way, they are hoping to be viewed as having "humbly accepted" the praise, and they think that they can create this view by spouting about how humbled they are as they are accepting it.  

Really humbly accepting an endorsement is much shorter and quieter.  It does not make a good sound bite.  It goes like this:  "Thank you, sir, I accept."

Humbled rarely announces itself.  Occasionally decorum and closure requires it.  The proper kind of announcement that you are "humbled" is gravely, in response to a setback; it accompanies the sort of statement that is referred to as an "admission."

(Yeah, this was inspired by Sen. Obama's response to the news that Gen. Powell had released a statement endorsing him, but it's a generic nonpartisan complaint.  Everybody does this nowadays, it's not even limited to politicians.)