Both Recent Changes and Watchlist offer the ability to bookmark filter settings to the "Saved filters" menu. Because these pages have slightly different toolsets (e.g., Watchlist doesn't have the "On Watchlist" and "Not on Watchlist" filters), because users may well have different filtering needs on these two pages, and because the default settings on the two pages are different, we need to make sure that the two pages' "Saved filters" collections are separate.
The tools will work in exactly the same way on both pages. But when RC page beta users show up on day one to the Watchlist beta, they will find 1) their Saved filters menu is empty and 2) the "No links saved yet" message is displayed.
(Could a knowledgable user copy the URL parameters for a complex setting from one page to the other and then save them in his/her new location? It would be nice. But absolute query parameter compatibility is not required.)