This is true, because of compatibility issue. The auto firmware and software update only allows updates that are within your device specifications. Usually the device system scans the update and it is then determined if its applicable to your device. The standard of requirements may be set to optimum that's why some updates will not be downloaded specially when your smartphone is an old model.
But there is a way to manually update your device, this is where you replace the system coming from a flash tool.
Flashing or updating using a flashing tool, is common nowadays. Primarily it is used for troubleshooting purposes. But because of its capabilities, it is now being used for updating and editing.
Depending on your device, you can search dozens of flashing tool on the internet. Just make sure that you read and understand the instructions.
Just be aware that using this tools will revoke your user privileges if the unit is under warranty.
Have a great day