Yes, AJAX vs HTML web-client.
Unless you have zimbraFeatureIMEnabled FALSE on COS/account; but that's not your case as toggling screen resolution changes web-clients & displays IM (only available in AJAX) for you.
Docs & Briefcase aren't yet available in the HTML client yet either:
Bug 16842 - Enterprise: Support for docs in lite client
Bug 31473 - Documents for Html client
Bug 28647 - Enterprise: Support for briefcase in lite client /standard html client
Bug 31123 - Briefcase for HTML lite client
Phase 1: View-only
Phase 2: Simplistic modification capabilities