Unless there is an advantage for a brand to be highly salient during a particular period of time (e.g. during a seasonal peak in category sales) spreading out the media spend over time will provide a better return on investment.
In this example, BRAND A spends so much in a short spell that a lot of the advertising spend is wasted. The waste comes from creating additional exposures amongst people who have already seen the advertising recently since these exposures only have a small incremental effect on purchase intent (due to what's known as the 'diminishing returns' effect).
BRAND B, on the other hand, maintains higher saliency than its competitor for most of the year by spreading out its media spend.
For further insights about media planning, check out this short article or listen to this audio clip.