At least one batter is not out at the end of every innings, because once ten batters are out, the eleventh has no partner to bat on with, so the innings ends.
Usually, two batters finish not out if the batting side declares in first-class cricket, and often at the end of the scheduled number of overs in limited overs cricket.
Batting averages are personal and are calculated as runs divided by dismissals, so a player who often ends the innings not out may get an inflated batting average, on the face of it.
[3] Examples of this include MS Dhoni (84 not outs in ODIs), Michael Bevan (67 not outs in ODIs), James Anderson (101 not outs in 237 Test innings), and Bill Johnston topping the batting averages on the 1953 Australian tour of England.
[3] Using the formula of runs divided by innings understates performance for the following reasons: These counterbalancing elements have been at the heart of the rationale of keeping the existing formula (runs divided by dismissals) in the 21st century among cricket statisticians, who have used this method of collecting batting averages since the 18th century, after some intervening controversy.