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?

Consider,  for example,  the cyclic group $\langle a\rangle$ of order six. Then
$$\langle a\rangle =\langle a^2\rangle\rtimes \langle a^3\rangle=\{1,a^2,a^4\}\times \{1,a^3\}.$$
But the symmetric group
$$S_3=\langle (123)\rangle\rtimes\langle (12)\rangle=\{{\rm id},(123),(132)\}\rtimes\{{\rm id}, (12)\}.$$
So nonisomorphic groups can be written as semidirect product, whose corresponding factors are isomorphic!
The distinction, as Berci points out, is in conjugation. We have $$(a^2)^{a^3}=a^2$$ but $$(123)^{(12)}=(213)\neq(123).$$

I found this example in Roman's "Fundamentals of Group Theory: An Advanced Approach" early on in the section on semidirect products.
