What's behind the drop in Hillary Clinton's numbers?

Washington (CNN) - Three national polls out over the past two weeks indicate the same thing - that former Secretary of State Hillary Clinton's favorable rating is dropping.

But two things: If you're surprised by the slippage, don't be. And while edging down, Clinton's favorable ratings are still firmly in positive territory.

The decline in Clinton's numbers comes as some congressional Republicans continue to question what she knew and didn't know about last September's attack in Benghazi that left the U.S. ambassador to Libya and three other Americans dead, which happened on her watch as America's top diplomat. And the surveys come as Clinton, who if she ran for the White House in 2016 would instantly become the Democratic presidential front-runner, is increasingly viewed by Republicans as a political threat.

The new poll, released Monday morning, is from Gallup. Fifty-eight percent of people questioned in the survey say they have a favorable view of Clinton, down from 64% in April. And 39% say they see her in an unfavorable way, up from 31% two months ago.

According to a Bloomberg National Poll released Friday, 58% of Americans said they had a positive view of the former first lady and senator from New York, down 12 percentage points from December. And the proportion of those who said they have a very unfavorable view doubled to 22% from 11% during the same time.

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A Quinnipiac University survey released two weeks ago indicated Clinton's favorable rating had decreased nine percentage points to 52%, from 61% in February, when she stepped down as secretary of state.

The biggest drop in the new Gallup poll comes from independent voters, whose favorable rating of Clinton now stands at 52%, down around ten points from April. Only a quarter of Republicans have a favorable view of Clinton, a slight edging down over the past two months, with 94% of Democrats seeing her in a favorable light, up a slight 2 points.

It's not all too surprising that Clinton's rating would drop now that she's transitioned out of the State Department to a more political profile.

"Secretaries of state are usually seen as non-political figures, and Americans tend to ignore their own political views when judging them," says CNN Polling Director Keating Holland.

"Once Clinton left the State Department, it was inevitable that Americans would start to view her again in a political light. This has happened to her before. When she was first lady - another job that most Americans don't view through a political prism - her favorable rating was in the mid-50s and low 60s. But once she began to contemplate a bid for the Senate seat from New York, her favorables dropped below 50%, and didn't reach the 60% mark again until she was nominated as secretary of state," adds Holland.

The Gallup poll was conducted June 1-4, with 1,529 adults nationwide questioned by telephone. The survey's overall sampling error is plus or minus 3 percentage points.