Same difference again

Last school year I blogged about how children in year 4 were proving a generalisation about subtraction. It was time to try the same thing again with my new class. They took a little longer to get the task, but after a while all got to the stage of creating a set with a difference of three:

When I asked what they noticed, a few people saw that the set would continue forever:

But they didn't see mention how the pairs increment. So we left it at that.

When we'd earlier made equations about the number fifteen,

T had written this:

30 - 15 = 15

29 - 14 = 15

28 - 13 = 15

So I took it as my second chance, and a couple of days later showed the class the pattern. They got the idea quickly. What do you notice?

and

Everyone agreed with these generalisations and they went up on the wall.

I asked them if they were able to explain why these claims were true, using words and pictures. This year however they found it much harder to explain themselves, and I don't know why. Sure, the class has a different character. It's earlier in the year. But I'm puzzled that they found it harder to explain what they thought. Most managed to get something on paper, but it didn't seem to convey the general nature of the situation like it did last year.