  We systematically investigate the outflow structure of GW170817 in comparison
with a sample of 27 cosmological short GRBs by modelling their afterglow light
curves. We find that cosmological short GRBs share the same outflow structures
with GW170817, relativistic structured jets. The jet opening angle of GW170817
is $6.3^{+1.1}_{-0.6}{}^{\circ}$, which is consistent with that of cosmological
short GRBs ($\theta_0 = 6.9^\circ \pm 2.3^{\circ}$). Our analysis indicates
that GW170817 is viewed off-axis ($\theta_{\rm obs} = 30^{+7}_{-4}{}^{\circ}$),
while cosmological short GRBs are viewed on-axis ($\theta_{\rm obs} \lesssim
\theta_0$). The exceptional properties of the GW170817 afterglow can be
explained by the difference in observation angle alone. We demonstrate that the
light curves of the GW170817 afterglow, if viewed on-axis, are consistent with
those of cosmological short GRBs. Other properties of GW170817, such as Lorentz
factor $\Gamma \approx 150$, spectral index $p \approx 2.15$, isotropic
equivalent energy $E_{\rm iso} \approx 8 \times 10^{52}$ erg and interstellar
medium density $n_0 \approx 10^{-2}$ proton cm$^{-3}$, fit well within the
ranges of those of cosmological short GRBs. The similarity between the GW170817
outflow structure and those of cosmological short GRBs indicates that
cosmological short GRBs are likely neutron star mergers.
