Elements can be transported into a sample or leach out of a sample.
Scientists will reject theories about the age of the earth that do not conform to the norm.
They will argue that the clock was not reset if the age is too old, or that isotopes were selectively removed if the age turns out to be too young.
In the study on the Hawaii lava flow cited above, it was argued that entrapment of excessive amounts of argon gas had made the samples appear older than they were.
This assumption is backed by numerous scientific studies and is relatively sound.