Why is reputation change more than total reputation? Could not find info on this anywhere: following a notification I went to a link where one sees: $$ \begin{array}{ccc} +1206& 1,008& 861\\\ \textrm{change}&\textrm{total reputation}&\textrm{year reputation} \end{array} $$ How is this calculated? Seemingly it implies my reputation was around -200 at some point??

This number has nothing to do with the change _of points._ You did not mention the first line in the table, which is named 'year rank' and ranks users by the points gained in this year.

The subsequent 'change' refers to the change of this 'rank' (where a positive change, means a decrease of the numerical value of the rank, which written like this seems odd but is somehow quite reasonable). So, the +1206 means you gained that many positions in the yearly ranking (compared to the last year's one).

Look at some other users data, there this is quite more transparent. Moreover note that in the dropdown menu "Sort by:" the option containing change is called "rank change." For a more detailed explantion see How to read the User Reputation Leagues table?