It's not unique. It's because it's the last blessing in a series. The same is true for bone (b'rachamav) Y'rushalayim after a meal and the final b'racha of Halel and p'suke d'zimra.
Specifically, SA OC 215:1 has that we say amen to our own b'racha in each of these cases even without hearing someone else simultaneously, and I believe S'faradim follow this rule. Rama there limits that rule to bone Y'rushalayim.
Now, Rama there does not discuss the case of finishing the other b'rachos — Hashkivenu et al. — simultaneously with another's b'racha.
However, Aruch Hashulchan :4 notes that the Rama's limiting to bone Y'rushalayim is because that that's the only b'racha on which one must answer amen to his own (so as to differentiate the d'oraysa three b'rachos from the fourth): we don't answer amen to our own Hashkivenu et al. (where it's permitted but not required to do so) so as to lend more oomph to the differentiation afforded by answering bone.
This AHS supplies us, perhaps, with the reason answering one's own bone Y'rushalayim (and the others) is appropriate when hearing another's b'racha simultaneously. Since there's really nothing wrong with doing so (as there usually is when finishing a b'racha oneself), it's appropriate when one's heard someone else's b'racha.
As to why the last blessing in a series has the you-may-answer-your-own rule, I don't know.