How come this got approved right away despite there being a higher quality translated version already on the site?
Because you added the parent to it after it was approved. Unless the approver has already seen the image and remembers it, there's no way for them to tell it's a duplicate if it isn't tagged as such with the parent present. It's the job of the uploader to check for previous versions before uploading.
That said, this being a Pixiv JPG and the other being a Twitter JPG, it's probably up to someone with a higher resolution monitor than mine to decide which is of higher quality.
