A phone advertised with generous storage can refuse an update while claiming several gigabytes free. The discrepancy comes from several categories of consumption that the storage screen reports poorly.
Advertised capacity is not usable capacity
Storage is sold using decimal units while the operating system reports in binary units, which removes a noticeable percentage before anything is installed. Nothing has been lost; the two counts simply use different definitions of a gigabyte.
The system partition then takes a further share for the operating system, recovery images and preinstalled applications, none of which the user can remove or usually even see itemised.
The result is that a device sold with a round number of gigabytes presents meaningfully less on first boot, and that gap is fixed rather than something that can be reclaimed later.
Caches expand to fill available space
Applications store downloaded images, video segments, map tiles and prepared data locally so they do not have to fetch them repeatedly. This is deliberate behaviour that improves responsiveness.
Most caching implementations grow until either an internal limit or the device's free space forces eviction. An app that appears to occupy a modest amount can hold several times that in cached data.
Because caches rebuild themselves, clearing them produces a temporary improvement that reverses within days of normal use. The space was being used for a reason.
Media is duplicated more than users realise
A photo sent through a messaging application is typically saved again by the recipient app rather than referenced, so the same image can exist in several places at once.
Editing creates further copies, since non-destructive editing usually retains the original alongside the edited version so changes can be reverted later.
Video magnifies all of this, because high frame rate and high resolution capture produce files large enough that a small number of clips outweighs an entire photo library.
Updates need working room
Installing a system update requires space for the downloaded package and for the new system image while the old one still exists, so the requirement exceeds the size of the update itself.
Devices that maintain two system partitions to allow seamless updates reserve that space permanently, which is invisible in storage reporting but real in capacity terms.
Flash performance depends on free space
Solid-state storage writes in large blocks and must relocate existing data to make room, a process that needs spare area to work efficiently.
A nearly full device has less room for that shuffling, so writes slow down and the phone feels sluggish for reasons unrelated to processor speed.
This is why keeping a margin free improves responsiveness measurably, and why storage warnings appear well before the last byte is actually consumed.