Apple writes one update for all iPhones - they're designed to all use the same firmware. They don't announce a new update until it's been written, alpha tested, beta tested and is ready to be sent out. Then they announce that they're coming out with one soon.
With Android, Google announces a new update. Then it sends it to the manufacturers, who have to rewrite it to work on each of their devices. then the manufacturers send it to the carriers who make their own modifications. testing has to be done at each step. Once the update passes all the tests (and some manufacturers and some carriers let huge bugs slip through), it's released.
The time from the first meeting to discuss what's going into the new release to the time it starts getting to the phones is about the same in each case, it's when the announcement is made that's different. Jobs was very good at marketing - make the people think you can get an update out faster by keeping it a secret until after it's ready to release.