The slippery fantasy of mathhammer is a crazy attempt to bend the random processes of probability to the will of the 40k gamer. Why should this be the case, especially as many players build their armies based on these arcane calculations? Basically the mathhammer player will have calculated given the toughness, weapons skill or ballistic skill that with so many models or with these or other upgrades the chance of a group of individual dice rolls getting a beneficial results is mathematically uprated. The assumption made in these mathematical systems is that probability smooths out over a certain number of dice rolls, say 20 or more rolls for a squad firing. This assumption is physically false in this universe at least. The rolling of dice is actually a self organising system, chaos theory. Each dice that is rolled has an individual probability or hitting a particular value, but the number of dice rolled at any one time does not influence the result on each individual dice. Self organising systems have specific characteristics which include the concept of avalanches. What this means for the game player is runs of good luck or more commonly runs of bad luck during the span of a game regardless of the mathhammer that has gone before. The averaging of dice rolls undoubtedly happens but only over the life span of the universe. Not over the span of a game of 40k. In the meantime dice rolls in individual games will defeat the odds again and again.
Mathhammer is therefore only a bit of headology that can give one or another player an advantage as long as they believe in mathhammer. At the end of the day a game of 40k pits on entity against another as does a battle pit one general against another.
There are undoubtedly other ways of applying a bit of headology to opponents quite legally through modelling, basing and playing skills.