Top tier and tier 1 are generally considered one in the same, tier 1 as opposed to tier 2. It is still mostly marketing since the EPA standard is the supposed tier 2. The website you linked to is not an EPA site, it's a Top Tier promotional site. I will gladly be proven wrong, but I maintain that Top Tier (tier 1 or whatever) is an industry self standard not a government standard and therefore is suspect and can be inconsistent, as the OP claims.