It wasn't first class, nor would it have pretended to be. It was shorthaul business class, in a pretty standard European configuration.

Often, convertible sections are already set at a deeper pitch than normal economy. (This is very nice if you're flying economy but sitting in the convertible section. You may have a 3-3 configuration, but it's at business class pitch.) Some convertible seating changes from 3-3 to 2-3 by narrowing the row one side but making the 2 remaining seats wider, and widening the overall width of the row on the other side to make three seats, each wider than the economy seats.

Why would anyone pay extra for this? Usually because the shorthaul business class product is a complete package that includes features like lounge access and decent catering. The wider seat is but one part of the package. Others may have more accurate information about this, but it seems to be generally thought that business class shorthaul pricing within Europe is more realistic than the published first class shorthaul fares in the US, although under severe market pressure at the moment - and it's generally more difficult to get access to the business class cabin other than by paying a business class fare.