inner semidirect and outer semidirect relationship https://kconrad.math.uconn.edu/blurbs/grouptheory/group12.pdf.
In the text above, the author explains how to find all groups of order 12.
He does so by showing that a group $G$ (of order $12$) is isomorphic to the semi-direct product of the $2$-sylow ($P_2$) and $3$-sylow ($P_3$) subgroups.
Then, he splits to cases and goes through all the possible combinations of semidirect products of $P_2$ ($\Bbb Z_4$ or $\Bbb Z_2 \times \Bbb Z_2$) with $P_3$ ($\Bbb Z_3)$.
Now my problem arises: he also looks for all possible actions $P_3 \to {\rm Aut}(P_2)$ (or $P_2 \to {\rm Aut}(P_3)$).

But why is this necessary?

He proved that $P_2 \cap P_3$ is trivial, $|P_2P_3| = |G|$ and that $P_2$ is normal or $P_3$ is normal. This means that $G$ is isomorphic to the inner semi-direct product, so why there could be more nonisomorphic semidirect products?
An example of what I'm trying to ask: 
Let's say we're in the case that $P_2$ is normal and isomorphic to $\Bbb Z_2 \times \Bbb Z_2$, he found that $G$ is isomorphic to $\Bbb Z_2 \times \Bbb Z_2 \rtimes \Bbb Z_3$ or $G$ isomorphic to $\Bbb Z_2 \times \Bbb Z_2 \times \Bbb Z_3$. 
But, we already know that $G$ is isomorphic to the inner semidirect product, so how is it possible that there are two options but not one?

For any action $\varphi:G\to {\rm Aut}(H)$ we obtain a semidirect product by defining multiplication on elements of $G\times H$ by
$$(g,h)(g',h'):=(gg',\,\varphi_{g'}(h)h')$$
where conjugation $g^{-1}hg$ becomes $\varphi_g(h)$ because
$$(g^{-1},1)(1,h)(g,1)=(1,\varphi_g(h))\,.$$
For distinct actions, we can indeed obtain nonisomorphic semidirect products of the same two groups. For instance, if $\varphi$ is the constant map ${\rm id}_H$, then we obtain the ordinary direct product as a special case.
