It might seem like a small difference, but it's being offered as a sort of compromise between the current regulations and the original four-cylinder proposal.
The new V-6 plan also pushes back the start date for the smaller engines to 2014. The idea is to get all of the current teams to agree on a standard so the series doesn't lose any of its relatively few participants.
KERS hybrid systems are also part of the new proposal.
The 1.6-liter V-6 plan has already been approved by the FIA, and will soon go to a vote of the World Motor Sport Council.