Actually, wheat, pumpkin, melon, netherwart, reed and anything I may have missed don't drop a seedbag when they are GGR 1/1/1.
To be precise: Every plantable not-Seedbag item is assumed to have a stats value of 1/1/1 upon planting. Accordingly, all plants with 1/1/1 values will drop said seed item instead of actual seedbags. There are exceptions, f.e. Reeds base-value is 3/0/2, i think.