They cant. PCGS used to have a policy of tradding in th tags for 50cents which helped.
therefore the numbers you see are inflated. A few folks have corrected PCGS on the number, (I think wondercoin had a thread a while back about a error in the pops).
There are many rarer dated coins and types where the pops are not all that inflated. That data can still mean something with respect to total graded. There are also some rarer dates from the 19th century where you could probably buy every UNC that appears on the market and keep them from getting slabbed. The pops in this case would be overestimating the rarity of the coin in comparison to other dates.
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therefore the numbers you see are inflated. A few folks have corrected PCGS on the number, (I think wondercoin had a thread a while back about a error in the pops).
Rich
roadrunner