temporary vignette sticker is longer than 30 days

Hi Guys, with some reasons, my wife applied for a 3 months dependent visa under my T4 visa and she obtained the temporary vignette sticker valid from 18 of April to 29 of June which is the expire day of my T4. This is not like rest of you which valid for only 30 days. Under this situation, I would ask can my wife come to UK later than 17/05 which is 30 days after 18/04?
It quite confused me. Many thanks

It was probably issued like that due to the expiry of your visa being so soon.