Congress' official scorekeeper has downgraded the number of people it expects to buy Obamacare plans this year, saying many instead will purchase coverage outside the new insurance marketplaces.

In a new budget outlook released Monday, the Congressional Budget Office said 13 million people are expected to buy plans through the Affordable Care Act's exchanges this year, down from 21 million customers it projected last year. Eleven million of those covered will be eligible for need-based subsidies while 2 million won't be eligible, the agency estimated.

The new numbers are closer to enrollment goals set this year by the Obama administration, which said it hopes for 10 million paying Obamacare customers at the end of the third enrollment season. Officials set their goal much lower than what the CBO had projected in March, when it said an estimated 21 million would sign up in 2016.

The budget office said Monday it has downgraded its enrollment estimate partly because it expects more people not eligible for the income-based subsidies to buy plans outside the Obamacare exchanges. In March, the agency will release a fuller update of its enrollment expectations.

The agency also said that over the next decade the federal government will spend 4 percent more on Medicaid, the health insurance program for the poor, than it projected in August. Now, Medicaid spending is expected to reach $187 billion over the next 10 years.

That's because CBO now expects more spending on newly eligible Obamacare enrollees than originally anticipated, saying 14.5 million Americans eligible for Medicaid under the healthcare law will enroll in the program in 2025 instead of 11.5 million as last estimated.

Obamacare enrollment ends in just six days, on Jan. 31, unless the administration prolongs the signup season due to a blizzard in the Northeastern U.S.