  Radiative transfer models of two transitional type Ia supernova (SNe Ia) have
been produced using the abundance stratification technique. These two objects
--designated SN 2007on and SN 2011iv-- both exploded in the same galaxy, NGC
1404, which allows for a direct comparison. SN 2007on synthesised 0.25
$M_{\odot}$ of $^{56}$Ni and was less luminous than SN 2011iv, which produced
0.31 $M_{\odot}$ of $^{56}$Ni. SN 2007on had a lower central density
($\rho_{c}$) and higher explosion energy ($E_{\rm kin}$ $\sim
1.3\pm$0.3$\times10^{51}$erg) than SN 2011iv, and it produced less nuclear
statistical equilibrium (NSE) elements (0.06 $M_{\odot}$). Whereas, SN 2011iv
had a larger $\rho_{c}$, which increased the electron capture rate in the
lowest velocity regions, and produced 0.35 $M_{\odot}$ of stable NSE elements.
SN 2011iv had an explosion energy of $E_{\rm kin}$ $\sim 0.9
\pm$0.2$\times10^{51}$erg. Both objects had an ejecta mass consistent with the
Chandrasekhar mass (Ch-mass), and their observational properties are well
described by predictions from delayed-detonation explosion models. Within this
framework, comparison to the sub-luminous SN 1986G indicates SN 2011iv and SN
1986G have different transition densities ($\rho_{tr}$) but similar $\rho_{c}$.
Whereas, SN 1986G and SN 2007on had a similar $\rho_{tr}$ but different
$\rho_{c}$. Finally, we examine the colour-stretch parameter $s_{BV}$ vs.
$L_{max}$ relation and determine that the bulk of SNe Ia (including the
sub-luminous ones) are consistent with Ch-mass delayed-detonation explosions,
where the main parameter driving the diversity is $\rho_{tr}$. We also find
$\rho_{c}$ to be driving the second order scatter observed at the faint end of
the luminosity-width relationship.
