I think I've figured it out, the two groups were actually looking at the same interrupt, so editing one edited both.
I fixed it by just making a second interrupt and changing it, rather than changing the existing one.

Maybe when you're editing an interrupt, it should say how many of each train ...

I've got two general train groups, one for fluids and one for items.

When I set the interrupts for one group, the other also seems to change which breaks it.
I don't know if this is something to do with using the parameter in the group name, or if I'm just not understanding the train groups ...