I don’t have an exact explanation, only noting that on recent versions of macOS, it’s definitely possible for macOS and applications to disagree on the amount of free space, because of how they might account for things like swap/temp files, APFS snapshots, and other purgeable data.
Explainer: Disk free space
Where did all that free space go on my APFS disk?
…free space available now doesn’t necessarily represent the amount of space committed to the items already stored on that disk…never try estimating free space by totalling the file sizes of a volume’s contents.
…APFS is a more complex and dynamic file system than HFS+, in that there often isn’t a simple answer to how much free space there is on any given disk or volume…This doesn’t always solve problems with installers and apps though…A few just don’t seem able to get their estimates of free space right, in which case you’ll need to inform the developer and ask them for a workaround.
If this is a case of Lightroom not properly estimating free space, this could be submitted as a bug in the Bugs section of this community.