“Simpson’s paradox, or the Yule–Simpson effect, is a phenomenon in probability and statistics, in which a trend appears in several different groups of data but disappears or reverses when these groups are combined. It is sometimes given the descriptive title reversal paradox or amalgamation paradox.”
This seems counterintuitive, but the 5 minute video below explains the concept well.
Wikipedia: Simpson’s paradox
Minute Physics: Simpson’s Paradox