it is very clear to me. Its about grammar issues really. The guy did exactly what the wife said. Often we assume that we have communicated what we mean, but in real sense we have not. in this case the woman gave wrong instructions. I would have done exactly that. The woman did not ask for any avocados, she only told the men to check "if"there were some, upon which if the check was a positive, he should increase the quantities of milk to six. She should have ended the sentence as …“get six OF THEM”.
It is a poor attempt at humour. Anyway imenikumbusha a colleague of mine whose wife had a tendency ya kumtumia orders kwa SMS. She would start at 2 pm ati sweet ukuje na pegs. At 2.20 anatuma ingine ati harpic imeisha, ooh sijui carrots hakuna. The texts would go on hadi 5 pm anamalizia na usisahau maziwa na mkate na usinunue ile maziwa ya jana :D. The guy would get so mad yani it was funny but after sometime I started sympathising with him.
One way to know you have grown old as a woman ni kuacha kuchagua mathree ati unataka nganya. And for a man ukishindwa kusimama kwa kibanda kuchagua avocado.
What makes the above joke humorous is
actually called zero anaphora or gapping.
[more explanation]
A wife asks her husband, “Could you please
go shopping for me and buy one carton of
milk, and if they have avocados, get 6 [gap].”
The gap leaves open the possibility of
referring back to either noun phrase,
“avocados” or “one carton of milk”. However,
it makes more sense to start the anaphora
resolution process by looking at the nearest
antecedent first.
Ambiguities of anaphora and reference are fairly
often the source of humour. An example from Literal
Minded :
Wife: Jim kisses his wife goodbye before he
leaves for work every morning. Why don’t you
do that? http://allthingslinguistic.com/post/50763364242/anaphora-jokes
Husband: Because Jim wouldn’t like it!