XCognitive Bias
Out-Group Homogeneity Bias
Your bias is Out-Group Homogeneity. Assuming members of other groups are all similar.
Why This Bias Happens
Distance reduces detail. We notice variety among those near us and flatten those far from us into one shape.
Example 1
Seneca speaks of common human nature beneath rank and nation; that reminder pushes us to see individuals, not caricatures.
Example 2
Research on intergroup perception finds people describe out-groups as uniform while describing in-groups as diverse.