When we start saying mashiv haruach there is an important requirement of announcing the liturgical change. If one does not hear the announcement, one does not say mashiv until the community would have heard it (this presents the relevant halachos). Also according to the linked Mishna Berura, if one makes a mistake, one does not repeat.
But when we change to vtein Tal, there is no such mention in halacha of a public declaration even though the obligation is seemingly stronger (in that one who misses it DOES repeat - here, second paragraph). I am not sure if my question is why there is an announcement at mashiv haru'ach or why there is none by vtein tal, but it probably includes "why are the two treated differently in terms of public announcement?"
P.S. This question, though I came up with it this morning, is strikingly similar to an older, unanswered, and better-worded question posted by Akiva Miller to the Avodah email list.