"Variance" is an important term in statistics and plays a crucial role in technical documentation as well. Without getting too technical about it: "variance" denotes the way the values of a set of elements vary around a central mean value.
Imagine you weighing a hundred marbles. Let's say the arithmetic-mean weight of an "average" marble is 10 ounces. But some weigh 10.1 ounces, some 9.8, some others 9.5 and 11.3, etc. You get the picture.
The "average degree" to which any given marble differs from that central value of "10 ounces" is called "variance" and is closely related to the statistical concept of "standard deviation." In short, "variance" is a measure of the extent to which the individual elements of a set are identical. "Zero variance" means all elements in a group are exactly alike.
In manufacturing, "zero variance" is a highly coveted but impossible goal because it means there are no manufacturing defects.
In technical documentation "zero variance" is equally desirable in this sense: there should be no syntax, vocabulary and style "variations" in your manuals. Why? Because a technical text that varies in syntax, vocabulary and style from page to page confuses the readers and fails to instill confidence.
You should have "zero variance" in your vocabulary. If you have used the term "LAN" on the first page, you should not switch to "local network" on page 10.
If you have used a Figure Caption AFTER Figure 1, you should not use it BEFORE Figures 2, 3 and 4.
If you have used "Arial 14 Points" for Header 1 on page 1, the Header 1 on page 56 should not have "Times New Roman 18 Points" font.
If the First Page template in Chapter 1 has a 1.5 inch left margin, every Chapter First Page should also have a 1.5 inch left margin.
You should aim for "zero variance" in your icons, table formatting, bullets, they way you use active and passive voice, and other style elements. "Zero variance" is one of the important features that make technical writing predictable, trustworthy and easy to follow. That should be a goal in every technical documentation project you tackle.