For years, the rate at which credit-card companies write off loan balances has shadowed the unemployment rate. An out-of-work borrower, of course, tends to fall behind on payments.

But for months now, some U.S. card issuers, such as American Express Co., Capital One Financial Corp., J.P. Morgan Chase & Co., Bank of America Corp., Citigroup Inc. and Discover Financial Services, are reporting improving credit trends despite stubbornly high unemployment rates. Issuers of plastic are also seeing a bump in earnings from whittling down their loss reserves, a trend that's expected to continue this year.

The reason for the divergence is grim: Some people have been unemployed so long they have simply been washed out of the credit system and no longer have any effect on the numbers.

"We have never seen the kind of divergence we've seen this time" between unemployment and credit losses, said David Nelms, Discover Financial's chief executive, last month in an interview. "I expect credit will continue to improve. I'm much less optimistic about the total unemployment rate."

The U.S. economy shed jobs in June for the first time this year and the unemployment rate remained high at 9.5%, according to data released Friday. A slight decline in the unemployment rate, from 9.7%, reflected discouraged workers giving up on searching for jobs. In a sign of the labor market's continued weakness, 46% of unemployed Americans were out of work for more than six months in June.

Meanwhile, the quality of credit-card loans has been broadly improving this year. Discover wrote off an annualized 7.97% of its card balances in its fiscal second quarter ended May 31, lower than the 8.51% rate in the first quarter. Write-offs in May similarly improved for most of its peers, who will report second-quarter results later this month.

For instance, AmEx's uncollectible U.S. card balances totaled an annual rate of 6.3% in May, down from 6.7% in April, and J.P. Morgan Chase wrote off 8.95% of card loans, down from 9.03%, during the same period. In a sign of things to come, the rate of borrowers behind on their card payments-a key gauge for future loan losses-is also falling across the board.

"I think we actually see in this phase of the cycle a little bit of reduction in the impact on credit metrics of things like unemployment," said Richard D. Fairbank, Capital One's chief executive, after the bank reported fourth-quarter results in January.

As Americans stay unemployed for long stretches, they fall behind on card payments, get written off-and have no access to new credit. While their continued unemployment keeps the jobless rate high, they no longer have any influence on the statistics of delinquencies or write-offs on cards. Meanwhile, card lenders have tightened standards, and new borrowers are less likely to get into trouble.

"Over time the weaker cardholders get weeded out," says Richard X. Bove, an analyst with Rochdale Securities, so "the quality of your portfolio gets better even as unemployment is high."

Card companies, burned by credit losses and sweeping credit-card legislation passed last year, have scaled back on credit lines, becoming more selective on borrowers. New rules limiting fees, such as those hitting card users exceeding their credit limit or paying late, and curbs on rate increases, make it less lucrative for card issuers to lend to the less creditworthy.

This time around, says Scott Hoyt, senior director of consumer economics at website Moody's Economy.com, the impact on credit losses "from actions taken by lenders has become stronger than the unemployment rate."

Write to Aparajita Saha-Bubna at Aparajita.Saha-Bubna@dowjones.com