Three short lines set two very different kinds of accounting side by side. The first is the human kind: a treasurer's neat ledgers, with their columns, totals and sense of order. The second is the hogs' kind, and it's far messier. They root around in the yard until they have dug out a hollow, a sort of cellar, where the ground has been turned inside out.
The poem never says what the speaker feels, but the joke is easy to catch. While official figures sit on paper, the animals are quietly making something real, or at least wrecking the yard in a way that can't be tidied into a column. The word "cellar" is a nice touch, since it's a place where goods and records are stored, and here the hogs have built one by accident.
The mood is dry and amused, with a rural, earthy feel. The dash between the lines works like a pause before the punchline, and the result is a small, wry picture of order versus appetite.