In brief
counter-increment changes a named counter's value. For example, counter-increment: chapter increments it on each element receiving the rule.
Useful to know
counter-increment changes a named counter's value. For example, counter-increment: chapter increments it on each element receiving the rule. [1]
Displaying the value is separate and uses generated content. If numbers skip, inspect which elements match the selector and where the counter is created. Two matching locations can add more steps than intended.
Sources
For further detail and to check the information: