Weighing the coal sacks

Text 1: My aunt at the gate, by Rosa Hemming

An original letter written in a nineteenth-century voice; a period simulation, not an archival document.

Each fortnight a cart brings our coal, and each fortnight my aunt stands at the gate with her arms folded until the carman has weighed every sack on the little iron scale hung from his tailboard. He grumbles that no other house on the road keeps him so long. Last November, however, a sack sold to us as a full hundredweight was found by the grocer next door to be short by nearly a fifth, and the merchant only laughed when she complained. Since then she has trusted the scale and not the label. The carman no longer hurries her; he has learned that the sooner the needle settles, the sooner he has his money.

Explain why the aunt insists on watching every sack of coal being weighed.