Autumn planted onion sets interesting

So last October I was in the GC and saw onion sets for sale. I had never tried them before but the assistant said they grew very well so I bought a bag of Sturon and Red baron . Planted them out and left them to get on with it. We have mild winter weather by UK standards and they both developed very well. Now at the end of June the sturons have done very well and I have some really lovely onions just nearing the ripening stage but the red baron have been at total failure and nearly all have bolted. Out of about 80 of each variety only 6 red baron did not bolt while only ten of the sturon did bolt.
So I am wondering is this perhaps due to red baron bing more sensitive to the cold and sensing it is in its second year of growth. I know you always get bolters with onion sets but that is an extreme contrast. So is this the case or have I just been unlucky with the red baron sets? I will grow the sturon sets again this coming autumn but the red onions are out