Top Tier is simply the recognition that the gasoline has appropriate levels of detergent through all octane levels, not just the highest octane.

ExxonMobil, BP, etc may have sufficient detergent levels in their highest octane but not the lowest (per the manufacturer group).

This may be a marketing gimmick, but they do have standards:
http://www.toptiergas.com/deposit_control.html

I only use Shell or Chevron/Texaco in my area