Hi Micha,
Well, both 'flavors' of Attribute are Product Attributes, but as you say, one has values that are product-specific, i.e. they don't usually change for that product, like 'Price', and the other is project-specific; the values will usually be different for the same product e.g. 'Room'. For the Project-specific attributes these are added as Placeholders, i.e. an empty space to add a value, by the application; it's an application-level action. But for the product-specific values you want somewhere to add your default values (like 'Price') so this is why these Attributes are stored in the excel spreadsheet, and why the project-specific ones are not. It doesn't make sense to add cells to the spreadsheet if it doesn't make sense to enter values in those cells.
The display of Attributes in the Properties Grid is sorted alphabetically, so you cannot change the order of display, unless you do so in the name e.g. "01 - Price", "02 - Cost".
Putting library products and UDPs that have the same manufacturer into the same tab in the Product Attributes spreadsheet is by design and makes more sense, in our opinion, that having a different tab just because the product happens to be a UDP.
I hope this helps to clarify things.