I think the original idea was that guardinals deal with problems and don't make a show of it, which is pretty NG. If you look at what outsider N's on the L-N-C scale actually do (as opposed to what the books say alignment means for PC's), you will notice that N's tend to have a strong "don't look at us too closely" thing going. The basic Rilmani story is "young king fights evil guided by mysterious advisor, when he is successful, the advisor betrays him (the advisor is secretly a Rilmani) because good is as unbalanced as evil"--in other words, the Rilmani doesn't want you to think about it. The classic Yugoloth story is that the Yugoloth needs to look dependable enough that someone will hire them as a mercenary, but not so dependable that someone else won't hire them to betray their first employer--again, don't look at me too closely. The Guardinal comes into town, kills off the big bad, and leaves. It doesn't make a big Captain America or Adam West Batman speech about "this is justice" like a LG type would and doesn't want a big party like a CG would. The townsfolk might not even know the Guardinal was a celestial.
It is kind of like the Celestial video, where they basically said, "if there is no malice, it isn't evil, it is neutral", for the G-N-E scale. That is not what the alignment for PC's advice says in PHB, but it is completely consistent with what N outsiders do. When the slaad tadpole bursts out of your PC's chest, the slaad that put it there isn't laughing about how the PC spasmed when it happened (unless you have a really mean DM).
