If I catch it right, somewhere along the line before the launch of the Blade and Reference Metas there's been a bit of a paradigm shift in KEF's engineering target, and now the goal is to provide as smooth of a sound power response as possible, while permitting some On-Axis/Listening Window deviations, hence the on-axis slope.
I would assume that this change is to accommodate reflective, domestic rooms and improve listener preference in these environments, but if one feels dissatisfied with this change, or have a well-damped, acoustically dead listening room, it should be relatively easy to reshape the response to provide a flatter LW, as directivity is fairly smooth and well-controlled in KEF speakers in general, it also didn't really change that much between generations, as the enclosure design remains unchanged.