If we'd been earlier in the lifecycle then this particular issue would almost certainly have been considered a bug but at this stage it is very unlikely that anyone will even contemplate doing anything about it, which means it's a feature that we'll have to live with and hopefully see gone once DW2 is released in a few years.
And yes, I am fairly sure that pretty much nobody hoping to make a living out of building games would intentionally present different information on two screens that ought to have the same information in them, at least not without even a trace of ingame explanation. Doing so would be bizarre beyond words.
As for what it is, I don't think we ever found out, but it seems to be affected by corruption and happiness. Even so, the colony info is supposed to take both of those into account in its displayed information, and during the start of the game there isn't any problem at all, but once you expand a bit and have a whole bunch of clonies, it suddenly kicks in. Furthermore, this thing affects not just the tax revenue but also the actual colony production (GDP). You can tell easily by going through your colonies and sum up their GDP and then comparing with the colony revenue figure in the summary screen.
I wish the devs would weigh in with some detail so we at least had a clue what was going on, but I doubt we'll see that happening either.
Oh, one more thing. It is also present in normal difficulty. I tested.
< Message edited by Spidey -- 8/3/2015 11:06:35 AM >