Lead levels, revisited

Justin and Timothy had their lead levels re-tested with a blood test. When they had the finger-stick tests, both came up with levels of 10. They want kids to be under 10, although no amount of lead in your body is considered safe. If a finger-stick test comes up with a result of 10 or over, then you have to retest with a blood test, because finger-sticks have a higher risk of contamination.
So on May 6 both boys did the blood test again. In about a month, Justin had gone from a 10 to a 5, and Timothy went from 10 to 7! What is the difference? Since the house was tested I don't let them on the third floor or basement or porch. I'm not 100% successful because they do sometimes wander down when I try to quick switch laundry into the dryer or when we leave the house Timothy will run over to climb on the porch railings. But their exposure to those three areas has been greatly reduced. Jacob's due to be re-tested again at the beginning of June, it should be interesting to see where his lead level lies. (also interesting that now that Jacob's level is at 19, he's only being tested every 3 months but when it was only at 14 they had a fit because they wanted him tested every single month!)