Fact: Some owners have customer accounts.
Fact: Some AEs require help completing their forecasts.
In response to these two truths we have supplemented our original Forecast Editing Tools. A User can hold one or both of these tools.

Edit My Forecast: Displays only those Projects for which the User is the Primary Contact for the Customer User associated with the Project as External Project Owner One.
Edit Sales Forecasts: Displays any Project where the User is a member of a Group for the External Company associated to the Project.
Bottom line: Edit My Forecast is intended for the individual AE, whereas the Edit Sales Forecasts will allow management or administrative support help the AE manage their Forecasts.