The estimate seems to come form questions 13 and 14 put together.

13. Do you think the number of U.S. military forces in Iraq should be
(increased), (decreased), or kept about the same?

38% said decrease

BASED ON THOSE WHO ANSWERED "DECREASED" IN Q13
14. Should all U.S. forces in Iraq be withdrawn immediately, or should they be
decreased, but not all withdrawn immediately?

34% said widthdraw immediately

34% of 38% comes out too 13% or 1/8 of the total.

but to use that number is deliberately misleading, 38% of the total are still saying we should leave, the only difference is between pulling out graduly or all at once.