Searches with time frames of issue

I note that sometimes putting brackets of a time period of issue loses stamps, which fall within that time period but are eliminated in the search. For instance, set the period to 1840-1940 (not terribly original) and search on South Africa. You catch some Scott #23’s but not all. Other times, the search reports out stamps that come after the set time period (e.g., it reports out SOUTH AFRICA -CISKEI Mushrooms). Help me; what is going on?
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