(AP Photo/David J. Phillip)

The Obama administration on Thursday issued a plea to Obamacare customers returning to the law's health insurance marketplaces: shop around if you want to get a better deal this year.

More than 70 percent of people currently enrolled in Affordable Care Act health insurance marketplace insurance can find a 2015 health plan offering the same level of coverage at a cheaper premium, according to a report from the Department of Health and Human Services. Almost of 80 percent of current enrollees could find a health plan with a monthly premium lower than $100 after tax credits are applied, HHS said.

"For some consumers — we think a large number of consumers — affordable premiums are going to be their number one factor," said Andy Slavitt, principal deputy administrator of the Centers for Medicare and Medicaid Services, on a Thursday press call.

Meanwhile, premiums for a key coverage level, known as the benchmark plans, are increasing just 2 percent on average this year, according to the administration. But many customers who bought low-cost plans last year may receive an unexpected premium hike this year if they stick with their coverage.

The complicated dynamics of the benchmark plans highlight part of the reason why HHS is urging existing enrollees to shop around this year to avoid unexpected bills. The benchmark plan is the second-lowest cost "silver" plan in each area, and it's used to calculate how much of a tax credit a consumer can receive. The benchmark plan may have changed in each area — which means the size of the tax credit for each plan could change in 2015. So it's entirely possible that a person's premium could stay the same price in 2015, but they could have to pay more because the benchmark plan in the area is cheaper. The following map from the Kaiser Family Foundation shows just the year-to-year variation in these important benchmark plans. The administration's report doesn't address the potentially high out-of-pocket costs under the marketplace plans. The average deductible for individual bronze plans, which offer some of the cheapest premiums, increased from $5,081 in 2014 health plans to $5,181 in 2015, according to a Health Pocket analysis. The average silver plan comes with a $2,927 deductible in 2015, which is similar to 2014 levels, according to the group.

The HHS report comes almost three weeks into the start of Obamacare's second-ever enrollment period. Shoppers have until Feb. 15 to pick a health plan, but they only have until Dec. 15 to choose a plan for coverage starting Jan 1.

Previous research shows that people generally stick with their health insurance, even if it means passing up a better deal. But administration officials over the past few months have been urging consumers to come back to the improved enrollment Web site and shop around. They're also sending "highly targeted" communications to existing enrollees "when people are in situations where more information will help them," Slavitt said.

Administration officials also stressed that the online renewal process for those that want to stay with their current coverage can usually be completed in less than a half-hour. "Even those people, we strongly encourage to come back to the Web site, update their income information and make sure they're maximizing their tax credits," Slavitt said.

About 6.7 million people were enrolled in marketplace coverage at the start of the ACA's second-ever enrollment period. Between returning customers and new customers, HHS said it hopes to enroll 9.1 million in marketplace health plans by the end of the current enrollment period.

Through the first two weeks of enrollment, HHS said about 765,000 people have selected 2015 health plans through HealthCare.gov, the enrollment portal serving 37 states. People enrolled in 2014 health plans make up 52 percent of shoppers so far, according to HHS figures.

The department started issuing weekly enrollment reports last month after it said it erroneously included about 400,000 people who bought just dental coverage in two previous enrollment updates.