[pxc]

1:3 (one to three) odds aren't the same as 1/3 odds (1 in 3 odds).

1:3 means the second outcome (that on the right-hand side of the ':' symbol) is 3 times as likely as the former outcome, which is true when the likelihood of the first outcome is 25% and the likelihood of the second outcome is 75%.

1/3 describes the chances only of one of the outcomes, and fixes it at 33⅓%. If there's only one other outcome, its likelihood is 66⅔%.

Is the 'addition' you're talking about just one of the readings of the first syntax, or did I miss something else in the video that made the initial presentation of those odds figures surprising?

Sorry if my terminology is off; it's been a long time since I did any stats