There is a Cartan decomposition for $PGL_2(\mathbb{Q}_p)$ meaning that double coset of the from $PGL_2(\mathbb{Q}_p) //PGL_2(\mathbb{Z}_p)$ are represented by $\alpha^k$, $k\geq0$.
So this is the reason because the definitions are equivalent.
There is a Cartan decomposition for $PGL_2(\mathbb{Q}_p)$ meaning that double coset of the from $PGL_2(\mathbb{Q}_p) //PGL_2(\mathbb{Z}_p)$ are represented by $\alpha^k$, $k\geq0$.
So this is the reason because the definitions are equivalent.