Tony. Not to do with exit pressure..
First consider this is a 4-2-1 exhaust manifold and Consider the exhaust gas from each cylinder to be a set body size.. I. E a 2 inch glug traveling down the pipe.
As that glug of exhaust gas exist the cumustion chamber and starts heading down the pipe it creates a mini vacuum behind it. After a known time period (will cover this in a moment) the glug gets to the exhaust section where it joins I. E. The first 2-1...
As this exhaust gas glug merges with the other joining pipe that vacuum is shared with both header pipes. At exactly the moment this occurs, the exhaust valves from the other pipe opens.
This means that the sudden mini vaccum 'sucks' the exhaust gasses out the cylinder a bit quicker than normal.
There is a significant amount of information about tuned exhaust pipes and it is incredible how critical gas flow is.
If this is in to a turbo, then having eqaul length pipes allows a smoother turbo spool and is less likely to suffer with minor boost fluctuations.
However you only need to worry about this if your chasing high and reliable power