Talk about the ultimate in "It's not you, it's me."

Close to half of employees who say they're actually happy with their jobs are nevertheless thinking of quitting.

That's according to a new survey from global consulting firm Mercer of 3,000 employees representing a cross-section of the U.S. workforce.

Overall, the survey found 37% of employees - happy or not - said they are seriously considering leaving, up from 33% back in 2011.

That's not surprising given that the economy has improved a lot since then.

But what is surprising is that such a high percentage of those who say they're happy at work may quit anyway, upending the going assumption that employees who are engaged at work are more likely to stay put.

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"The new twist is that the inclination to leave is increasingly detached from employees' satisfaction with jobs, pay and even growth opportunities," said Patrick Tomlinson, a talent leader at Mercer.

Indeed, among the group that said they were likely to move on, between 42% and 48% said they were happy with their organization, their job, their pay, their benefits, their opportunities, and management.

Unfortunately the survey did not actually ask respondents the obvious question: "So if you're happy, then why are you leaving?"

Maybe some of it has to do with an increasing sense of job insecurity. A full 42% said they were "concerned" or "very concerned" with losing their job.

Maybe some of it has to do with the need for change after rising up the ladder. Senior managers (63%) were more likely to be contemplating an exit than middle managers (39%) and non-management employees (32%).

Or maybe it just has to do with age and the fact that younger workers are still exploring life's opportunities. Millennials were the most likely (44%) to say they were seriously considering leaving versus 29% of those between the ages of 50 and 64.