What it means is that the seller rounded the figure of 138 to 140. But the sheep dog rounded them up (he chased them into a bunch - another way of saying that is rounding them up) and he makes 138. It's all on the words rounding up.
No, what it means is that the figures have been put the wrong way round ;D
A corrected version below.
A shepherd is out with his sheep and his sheep-dog... Later, he says to his sheep-dog,
"You look after these sheep while I go and pick up the mother-in-law. Tell ya' what, you count these sheep while I'm gone and tell me how many there are when I get back."
The sheep-dog complies and the shepherd sets off. When he gets back, he asks his dog,
"Alright how many were there, then?",
to which the sheep-dog replied, "There's a 140 of 'em."
The shepherd then said,
"What? That can't be right, cause when I went to the market this morning to get 'em the seller told me there was 138 of 'em!"
The sheep-dog then said, " Yeah, there are, but I rounded 'em up for you!"
