It's answered (I believe) right below the chart: "Importantly, the data does not solely include TEUs arriving in a particular week. It also includes TEUs arriving in prior weeks that the port expects to handle in the stated week."

In other words, it includes the backlog that's building up. "Expects to handle" isn't the same as "can handle." That's going to go up until the backlog is cleared.

(There is data that could prove me right or wrong, but the article doesn't provide it.)