The link refers to the 767-300ER Vers 2 which is tagged as a 76G (not 76C) on seatguru. Your headline references 76C which is actually a 767-400ER, Vers 1.

So I'm going with the seat situation you described which is the 76G.

Seat guru also says this at the top:
"Delta is replacing the BusinessElite seats on six 767-300ERs with Thompson Solutions Vantage flat-bed sleeper seats, like those found in the new Boeing 767-400ERs. Delta is also updating the Economy cabin with slimline seats and personal onDemand televisions. The first routes to be served by these reconfigured planes will be flying on longer sectors of 11 hours or more."

What's missing on seatguru at the moment is any reference to the new Economy Comfort seating Delta is installing in the first 4 rows of Y presumably at the same time they do the other modifications. This may account for the layout difference you noticed.

Are you looking at YOUR flight and seat choices? Or are you looking at Delta's seat maps for their aircraft?

If looking at YOUR specific flight, it may mean that your plane already has the new seating installed. Just be aware that any other amenities that come with it (e.g. free drinks) aren't happening (allegedly) until the full roll-out.

Still, if it were me, I'd grab the bulkhead EC seats. It's been reported in other threads that they are surprisingly spacious. ^

Hope this helps. Safe travels!