I'm not an expert, but I think some fare rules are written in a way that automated pricing engines can follow them, and in that case what Matrix or an airline or online travel agency shows as the price is accurate, but some rules are more complex and need to be interpreted by a person, generally a tariff desk agent at an airline. So, in this case, it may be that to get the lower fare you need to price it with an airline (or a travel agent that will send it to an airline's tariff desk). You could try using an airline web site and see if it prices it the way you think is correct.