Coverage is a subtle 😉 mix of the line and the branch coverage.
You can find the formula on our metric description page:
coverage = (CT + CF + LC)/(2*B + EL)
where
CT - branches that evaluated to "true" at least once
CF - branches that evaluated to "false" at least once
LC - lines covered (lines_to_cover - uncovered_lines)
B - total number of branches (2*B = conditions_to_cover)
EL - total number of executable lines (lines_to_cover)