Every different option selected may result in different production and delivery timelines.
Hypothetical examples:
The Burgundy cases may be from a different supplier than the Gray cases.
White cases are only produced once orders for nnn units have accumulated.
The 1 TB option may have memory from a different supplier than the 512 GB option.
The 2 TB option may only be produced on the first and third weeks of the month.
Etc ….
Plus keep in mind that iPhones are produced for specific regions with different hardware and software configurations.
And even the packaging is unique to each and every region in which the devices are sold.
The fact that all of these “moving pieces” come-together as efficiently as they do is actually quite amazing.