The numbers are different because the site doesn’t naively count every line but merges some as a single package. For example, at the very top of the Debian list we have 0ad, 0ad-data, 0ad-data-common. These are all counted as one single “package.”
One might argue that doing the comparison in that way is more useful to an average user asking “which distribution has more software available.”
I guess it depends how aggressively they merge packages… Some software has different versions which are all useful. Some software has multiple packages which are different things, for example a theming engine can have packages for various widget libraries and various versions (GTK3, GTK4, QT5, QT6) as well as an icon theme.
On the other hand, repos like the AUR (probably nix too) have outright duplicated packages, made by independent contributors.
