A csv file is generated under Mac. Open garbled with Excel under windows

if you want to convert it to GB18030 format, it won"t be garbled if you open it under windows

but it seems that the iconv command doesn"t work

  ~ head aaa.txt

  ~ file aaa.txt
aaa.txt: UTF-8 Unicode text

  ~ iconv -f UTF-8 -t GB18030 aaa.txt > bbb.txt
  ~ file bbb.txt
bbb.txt: ISO-8859 text
  ~ head bbb.txt


  ~ iconv -l|grep GB18030
GB18030

Why doesn"t the iconv command take effect? It"s supposed to be changed to GB18030. How did it become ISO-8859?