The Dev-team is probably trying their hardest right now to finish a major project, aka the expansion - publishing and correcting minor details like this in a presentation thats mostly meant to keep people excited and give them an idea about whats coming for them, is most likely not their biggest concern right now.
My guess: they continued to broadcast this information, that they claimed might be inaccurate because they simply had more important stuff on their plate than micromanage something silly like that.

The point in the live letters is not to get 100% correct information - seriously, you're complaining about something that might still be tested during that time to figure out how to best balance it. The point is to give people an insight into what the devs are doing, keep them excited, maybe a feeling of being "included". For all that we know those numbers are changing within the day while the battle team is trying something out - so giving one number and saying "that can change" is as accurate as it will get.
Final, 100% accurate information has to be in the patch-notes - and they referred you/everyone to those.

I get that people are upset with WHM-changes in general - what I dont get is why someone would get worked up like this over something that (in the grande scheme of things aka a whole expansion) is a minor detail...