It MAY happen if the 3A fills up faster than 2A since the price varies by demand. In this case that logic and concept is not applicable.
However, this is not the first instance of higher class filling up faster than lower class. In summer season in particular, 3A fills up faster than SL all the time all over India. This being a high demand sector (filled with business travelers), 2A might have filled up earlier. Not a surprise.