Url.Action will reuse the current request parameters, if you do not explicitly set them. It is by design in outbound url-matching algorithm. When looking for the route data parameters in a process of generating url, parameters are taken from:
1) explicitly provided values
2) values from the current request
3) defaults
In the order I specified above.
Outbound matching algorithm for routes is complicated, so it is good practice to explicitly set all parameters for request, as you did in your example