Grading

It's hardly an exact science but in general a TPG examines every coin for damage and defects. If they find something that they think meets their threshold for a "problem", the coin will be flagged as such (cleaned, damaged, corroded, etc.). That threshold varies depending on the coin type and year, the grading service and individual grader, the general level of condition, and possibly on phases of the moon. Who can really say? If minor problems are detected but the "reject threshold" is not met, the grader will apply a "net grading" logic and assign a lower numerical grade than the level of wear would otherwise warrant.

I do know that a coin rejected by PCGS for cleaning can make it into a no problem ICG slab, and a coin rejected one time by PCGS or NGC can make it into their graded slab upon re-submission. Probably every forum member who submits coins for grading can report similar discrepancies. It's the nature of the beast.

One consistant observation: coins in mint state grade are more likely to be called problem coins due to very minor cleaning. A somewhat worn coin with an equivalent level of cleaning will have an easier time getting into a graded slab.