By no means am I on the side of the WHO, but I'll try and play devil's advocate here.

How will most people interpret "we don't know"? You and I might understand the wisdom behind admitting to not having enough information, but the average person won't, and the media can easily mock this form of honesty.

Being knowingly wrong or inconsistent seems to be a better strategy since the attention span of the average person is rather short, and appeal to authority is a pretty effective mind trick.