I don't find it in the least bit surprising that the estimates and economic forecasts have proved over-pessimistic. It's standard practice in the financial world to take a 'prudent' view when forecasting - which means that you generally present a near-worst-case-scenario. This is on the basis, of course, that if things turn out better than forecast, your pessimism will be readily forgiven whereas, if you're over-optimistic and things go tits-up, you're up for a slaughtering from the critics.

Regarding the forecasts for the social and economic future of the U.K. after Brexit, only time will tell how accurate they are/have been.