Why does the Supernova 2006cm give a very different value for the Hubble constant? Why doesn't it increase error bars for the Hubble constant? The Supernova 2006cm has a redshift of 0.0153 which translates into a recession speed of 4600 km/s. 

It has a distance modulus of 34.71 which translates into a luminosity distance of 87 Mpc.

This gives a value of 53 km/s/Mpc for the Hubble constant.

Why doesn't this anomalous observation create sufficient doubt in the value of the Hubble constant?
At a distance of $d = 87\,\mathrm{Mpc}$, with a Hubble constant of roughly $H_0 = 70\,\mathrm{km}\,\mathrm{s}^{-1}\,\mathrm{Mpc}^{-1}$ cosmological expansion should make the host galaxy UGC 11723 recede at $v=H_0 \,d\simeq6100 \,\mathrm{km}\,\mathrm{s}^{-1}$.

However, galaxies also move through space, at typical velocities from several
$100\,\mathrm{km}\,\mathrm{s}^{-1}$ in galaxy groups (e.g Carlberg et al. 2000) to some $1000\,\mathrm{km}\,\mathrm{s}^{-1}$ in rich clusters (e.g Girardi et al. 1993; Karachentsev et al. 2006).

The observed velocity of $4900\,\mathrm{km}\,\mathrm{s}^{-1}$ (Falco et al. 2006) is hence $\sim1200\,\mathrm{km}\,\mathrm{s}^{-1}$ smaller than the Hubble flow, but quite consistent with what may be expected.

This is why supernovae at such small distances cannot be used to deduce the Hubble constant, unless a very large number is observed such that statistical errors cancel out.