You see 'them' as all alike, and 'us' as wonderfully varied

People reliably judge members of other groups as more similar to each other than members of their own group, even when the groups are objectively just as diverse. In one study, both younger and older adults saw the other age group as more homogeneous than their own — the bias flips depending only on which group is 'theirs.'

— George Quattrone & Edward Jones, The Perception of Variability within In-Groups and Out-Groups — Journal of Personality and Social Psychology, 1980