Who can explain the joke I don't understand?

The news of the two upstairs families may be that they have been operating for three generations, so they will be surprised to hear that it is two pence a catty.

I think it should explain the abbot's ignorance. He thinks that one kilogram of solid is more than one kilogram of bulk, but in fact it is as much as one kilogram. This is like someone asking whether a catty of iron is heavier or a catty of cotton, and someone will say that a catty of iron is heavier!