It is easier to understand if you imagine that they are "hearing" the gravitational waves instead of "seeing" the gravitational waves.
Analyzing how the waves affect both detectors and the delay, they can get a rough idea of the direction. The origin was in a single point that is probably in the dark zone in the bottom left.
I'm not sure of the reason of the shape, but resolving equations in sphere usually give curved solutions, and then projecting it to a planisphere add more distortion. [I'm interested to read a more detailed explanation. Also, why the longitude is in hours?]
It does come from a single point but they can not tell where inside the shaded area of space the single point was.
As far as I can tell from their website the event was seen by Virgo and only one of the Ligo detectors (Livingston), which makes localization in the sky rather uncertain. No clue at the moment why the second Ligo detector (Hanford) did not see it.
If not all of them see it and they have this shitty sky location, the one missing was either offline or its data was unusable for other reasons (ie someone left their phone on one of the mirrors and got a call).
If you had a working detector taking good data than not hearing it also tells you something. But if the detector was not taking good data for whatever reason (local seismic event, high surface winds, technical glitch), then it doesn't help you. Also afaik (and I am not an expert) the first automatic pipeline doesn't use non-detections.
I'm surprised that the gravity anomaly is coming from a large region of space. And it's a curve. Is that expected? Why is it happening?
A black hole merge is coming from a single point in space, so I would have expected there to be a single point. But I'm definitely missing something.