any unison effect works on the basis of detuning, but detuning in the realm of less than 10 cents ( or so, depends on sw)- the more voices the greater the variation in detuning. More than about 25 cents detuning starts putting you into the realm of quarter tone intonation.
Some examples:
2 voices in unison with the exact same tuning will sound thicker but not by much
Detune the 2 notes +2 cents and - 2 cents and it starts to get interesting because of the micro-pitch beating @ 4 cents total
4 voices and the detuning is now + 5 cents +2 cents -1 cent -3 cents even thicker line because of the beating between each pair of instruments: 8, 6, 5, 3, 3, 2 cents: lots of rich beating and inter-beating
8 voices and the detuning is now +7, +5, +4, +1, -3, -5, -7, -8: 15, 14, 13, 12, 12, 12, 11, 10, 10, 9, 9, 8, 8, 7, 6, 6, 5, 4, 4, 4, 3, 3, 3, 2, 2, 2, 1, 1 cents. Note the repetition of inter-note cent beatings.
the entire concept of unison effects comes from the interplay of tuning variations in violin sections in orchestras: there are those who can tell you which orchestra in which year is playing certain passages because of the individuality of the microtonal tuning issues within sections.
so whilst conceptually the notes are pitched differently they are all pitched around a central freq and so are deemed to be that freq with micro-tonal variations