Yes, it is normal if you have added the styles to the Geometry List after placing the trees:
By default Forest doesn't modify any parameter of the existing items (including ID), because is possible that the user would like to preserve the current distribution. To randomize the IDs after adding new styles, select the items and click "Random ID" followed by "Reset Size".
Instead, if you place new trees, they are randomly assigned from the full list. Also, if you want to plant trees using a specific ID, set a value in the "Geometry ID" parameter before using the "Create Trees" button.