The Gmail app on Android 2.2 seems to have a bug.
I saw an email from Twitter (about a person following me on Twitter), in my Gmail account via the web version of Gmail (using Opera Mini mobile browser), but the same email did not show in the Gmail Android app. Both checks were done on my Android phone, within seconds of each other, and there were newer emails that showed in both, so the issue may not be due to a delayed refresh in the app.
Also, lately, I've seen that many emails take longer to show up in the app versus the browser.
- Vasudev Ram