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Because M3/M4 only has half the current of M6.
The Vgs of M6 can therefore never be the same as that of M3.
This means that M3/M4 have different vds, and hence from channel length modulation, will never have identical current.
This is the cause of the offset.
BTW, this is not an easy question IMO to a student. Failing an exam because of this, your institution's standards must be quite high.