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Jason L"ultra high grade" takes into account how many similar cards there are in the highest grades, as opposed to "high grade", which simply refers to the high grades (7,8,9 and 10) themselves.
And just for the record, all of my cards are unleaded, and I do keep them under vapor-proof displays, so as to retard the evaporative processes that often plague a collection of minimal importance such as mine.
(Can you tell it's Friday after a choppy week in the market?) 