This is [meta-tag:status-bydesign], not a bug. Only posts with *negative score* can be deleted outright by trusted users; see the [trusted user privilege](http://meta.stackexchange.com/help/privileges/trusted-user):

> Trusted users are allowed to perform trusted actions, including:
>
> * Voting to delete answers **with score of -1 or lower**

Emphasis mine. In the Low Quality Review queue that limit is [raised to 0 or lower](http://meta.stackexchange.com/questions/230872/why-im-casting-delete-votes-in-the-low-quality-review-queue/230873#230873) to save you having to downvote first. But this still means that if the score of the post is 1 or higher, even trusted users only get to *recommend* deletion.

For example, I have 20k+ here on Meta, but for [this review](http://meta.stackexchange.com/review/low-quality-posts/30605) (post visible to 10k+ only) the post in question had a score of +2 when I voted to delete, so my vote was recorded as a recommendation rather than an outright deletion vote.