为什么RouteLink生成友好URL而ActionLink不生成?

考虑以下路线

routes.MapRoute("Routename1",
    "{someEnum}/SpecificAction/{id}/{stringId}",
    new { controller = "MyController", id = (int?)null, stringId= (string)null, action = "SpecificAction" },
    new { someEnum= "(EnumVal1|EnumVal2)" }
);

考虑以下两种链接方式:

<%=Html.RouteLink("Click me","Routename1", new { id = 32, stringId = "Yatzy" })%> 
<%=Html.ActionLink("Click me", "SpecificAction", "EnumVal1", new { id = 32, stringId = "Yatsy" }, null)%>

RouteLink生成正确的url,即/EnumVal1/SpecificAction/32/Yatzy

ActionLink生成一个类似于/EnumVal1/SpecificAction/32?stringId=Yatzy的url

为什么会这样?有人能给我解释一下吗。