The grammars in the tree-sitter package and nvim-treesitter package are
not the same. The grammars in the nvim-treesitter package are directly
taken from the grammars supported by the plugin, whereas tree-sitter
grammars are hard-coded in nixpkgs.
This means that there are more tree-sitter grammars available from
nvim-treesitter rather than tree-sitter.