Project forecast, period amount for preceeding periods when creating a new forecast dont match up.

Summary:

The preceeding periods Actuals dont match to the sum based on the previous forecast.

Content (please ensure you mask any confidential information):

There is a forecast for a project for Sept period:

The period amounts for preceeding period is  Actual Amount: 224706.66

The next period is Apr-25 and that has Actual Amount: 121616.64

 

A new forecast is created for Oct period:

Preceeding Period Actual Amount: 343323.30 (this is sum of previos forecasts preceeding and Apr-25)

Next period May-25 has actual of 6239.87

 

A new forecast is created for Nov period:

Preceeding Amount: 344141.81 (This is less than the previos Preceeding Period actuals + May-25).

What could be the possible reasons for this.