We numerically study the physical mechanism of the chirp compensation of the attosecond XUV pulses obtained by high-order harmonic generation. We show by detailed analysis that the macroscopic aspects of fundamental pulse propagation are essential in the formation and temporal-spectral-spatial properties of the attosecond pulses. Partial chirp compensation is already achievable due to the self-phase-modulation of the fundamental pulse. Further, by propagating the attosecond pulses in preformed plasma, we can fully compensate the inherent positive chirp of the attosecond pulses which were built up from the short electron trajectories in the harmonic generation process.