Why does League Two have more promotion places than the Championship and League One?

There’s something I’ve always wondered regarding the Football League’s structure:

• There are three teams promoted/relegated between the Premier League and the Championship;
• There are three teams promoted/relegated between the Championship and League One;
• But there are four teams promoted/relegated between Leagues One and Two.

This has been the case ever since the Football League was reorganised into four tiers in 1958-59. Wikipedia vaguely refers to this extra promotion/relegation place as a "tradition", but offers no explanation as to how or why this tradition began. I’ve never been able to think of an obvious reason why promotion/relegation between Leagues One and Two should be out of step with the leagues above them.

Why does League Two offer more promotion places than the Championship or League One? Is there a historical reason behind it, or was it determined arbitrarily?