That is when these projects were originally proposed. Maybe even earlier? I remember going to the Art Gallery for some opening for the other project Lamb proposed (I can't remember the other name).I sorted by permit date which brings up one's just entered into the system. There is no date on the one on open data. Where do you see 2015?
My guess is they are adding in older projects to open data (since the intent is to have all projects/applications), otherwise it would have the date on the entry for when it is submitted/approved.