The app store only checks for updates automatically once a week. When you open the app store manually it checks for updates right away, hence the badge showing up after you open the app.
Thx Prof Tripp-I've wonder this also why sometimes the update shows after opening up the app store.
On another note, I have another question that could be a bug also (?) it's happened to me twice on my iPhone and on a rare occasion on my iPad. When in the app store browsing on wifi, sometimes I will click into an app to get addl info on it if it interests me and I will get a pop up suddenly saying "cannot connect to app store" (or something along this lines) I had to go in and shut off my wifi, and then switch it back on--then it was fine.
Any one else experience this?
Sorry to derail your post OP but I fig'd this may be an app store bug also.