I can't speak for all the reviews nor how anyone else conducts editing review here or elsewhere, but for the ones I personally denied, some appeared to me to be subjective improvements. I was unable to definitively assert that the modification was an objective improvement on the question's title rather than rephrasing the information which was already there without making it especially more interpretable or descriptive of the question's content.
In two cases, at the time I was reviewing the edit the associated question was closed. In that context I performed the review on the criteria that any modifications would work towards addressing the reason for closure.
I would perform the review in the same fashion for any edit on any SE site, but I will admit that in instances such as these perhaps I've misinterpreted the guidance on editing review, in which case I would be happy to review any resources on editing guidelines! I definitely err towards the conservative in review tasks, but I would very much like to improve upon my approach if at all possible.
Addendum: On Title-Only Edits
On further reflection, I think that my review decisions in the context of seemingly subjective title edits have a lot to do with whether or not the edit consists solely of modifying the title. If the body of a post is modified or the tags corrected there's more to go on regarding whether or not an edit as a whole objectively improves a question - a title modification which I cannot confidently determine to offer an improvement is unlikely to be a factor in how I vote in that case.
But when the only edit is the title alone, then my vote on the edit is entirely determined by testing the changes to a few words against my understanding of "substantive improvement."
I think that determining the impact and appropriateness of a single change may arguably be more difficult than assessing the collective effect of multiple changes, which may play a role in minor edits seemingly receiving more volatile review outcomes across the SE sites.
Which, again, does not mean that I would assert that my interpretation of or contribution to edit reviews is absolutely in-line with the mechanism's intent or guidelines, but I wanted to share that reflection on my thought process. I will certainly read up on the guidelines and advice to try and find where I might need to course-correct.