If it was all the same batch, I can only think of a couple of questions... Were the jars dry? Did anyone "play with" the jars as they were setting?
I don't think temperature when opening the jars would affect it. Perhaps if the jelly was only *just* at setting point, the first few jars filled might be a bit thinner than the last few that had lost more water while waiting to be filled.
I should add I am just guessing here.