The phrase "to increase the variance" means to make the differences between different values larger or more diverse.
Sentences with «to increase the variance»
But if, in addition, global warming increases the variance of regional temperatures, then we increase the likelihood of extreme heat waves by a lot. (planet3.org)
That's because the trend not only increases the distance from the mean, it also increases the variance of the data. (tamino.wordpress.com)
It would appear that controlling for prior participation increases the variance accounted for in the models of both formal and informal social participation. (academic.oup.com)