The teacher was probably just looking for something that is appropriate at this point. If that's a Y6 sheet, then it's a Y6 sheet. I've got Y5 and often use KS3 resources (Levels 3-5 and 4-7) for mine. The level is the same, but it means I don't need to use the same resources as our Y6s. My kids also feel rather excited about doing "Year 7 and 8 work", especially when it's easy for them. When they find it hard, they don't get too wound up about it (which my more able tend to do, when they know it's something they should be capable of), because they know that it's really meant for older children.
Another factor this year is that many schools are starting the new curriculum for years 3 and 4. A lot of topics are now going to be covered earlier in a child's school career. Old year 5 and 6 resources will be reused for years 3 and 4.