The short term Mars missions do a non-standard path to get back. I believe many of them actually use a Venus flyby to get back to Earth, which you can see a bit on this Wikipedia article. In any case, one can return more quickly then waiting for an optimal transfer window, but at a much higher cost, even if there isn't a Venus flyby in the process. The mission generally is called the "Opposition" mission.
From the Case for Mars, which briefly discusses the plan, here are some dates. Both missions take about 6 months to arrive. The Conjunction mission has 550 days on Mars, the Opposition mission has 30 days. The Conjunction mission takes about the same time to return to Earth, 6 months, while the Opposition takes about 14 months return time. It also requires about 2 km/s more delta v to return.
The "Conjunction" plan has been presented in many mediums as the preferred plan to get to Mars, due to it's shorter mission duration. I believe the article you included is just flat wrong. The Case for Mars cites a 550 day on Mars mission being the standard. It seems you are showing there is about a 450 day window.
The journey probably won't occur at the exact optimum for a couple of reasons. One can do a faster mission to Mars by offsetting the mission time somewhat for only a small extra fuel cost. There are a number of "Free Return Trajectories" where one can return to Earth at little to no cost. The optimal of these will do the transfer orbit at a 2 year orbital period, which will cause it to return to Earth in 2 years if something fails. The time to get to Mars for such an orbit is 180 days. I believe these will get to Mars about a month earlier then the full Hohmann transfer orbit. Of course, the time on Mars varies depending on the exact orbital mission.
The other primary reason for not leaving at the optimal trajectory is you get there sooner then an optimal trajectory would do so. One could therefore land an unmanned spacecraft at the landing site if something should require that it be landed there, or could land it at the site of the next landing if that turns out to not be required.
Bottom line, the prevalence of the "Opposition" class mission in Mars planning probably lead to the error in the article you linked to. An opposition mission could be done, and would be almost a year shorter, but would spend most of that time in transit through the solar system, and less of it doing actual science.