Depending on how you define "optimal", there are usually two local minima in the impulsive trajectory characteristics for a given opportunity, called "Type 1" and "Type 2". The Type 1 trajectories go a bit less than 180° around the Sun, and the Type 2 trajectories go a bit more than 180° around the Sun. This bifurcation occurs due to the relative inclination of the orbit of Earth and the orbit of Mars. If the orbits of Earth and Mars were coplanar, there would be one local minima going 180° around the Sun.
This can be seen in the traditional "pork chop" plots, which show various trajectory characteristics, such as departure $C_3$ or arrival $V_\infty$, as a function of departure and arrival dates. Here is an example from the 2005 opportunity:
The Type 1 is the smaller region on the bottom, and the Type 2 is the larger region on the top.
TGO changed from a Type 2 to a Type 1, when they moved their launch out two months, but kept the same arrival date. TGO had a large deep space maneuver, so the pork chop plot doesn't have enough dimensions to represent the trade space, and the trajectories weren't the classic Type 1 and Type 2 impulsive trajectories (i.e. a single burn at Earth). But the basic principles still apply.