Aptanas’ Tab key handler takes precedence over Zen Codings’ one (btw, Zen Coding was renamed to Emmet recently: https://github.com/emmetio/emmet-eclipse).
So in your case, you’re actually expanding Aptana snippet, not Emmet one.
You should either remove all Aptana snippets to allow Emmet to handle them or configure (via Preferences > Keys) and use another shortcut to explicitly expand Emmet abbreviations.