>>400
POSIXでも、
The symbol $0 shall refer to the entire record; setting any other field causes the re-evaluation of $0. Assigning to $0 shall reset the values of all other fields and the NF built-in variable
と明言してるね。$1〜$nを変えれば$0が再構築され、$0を変えれば$1〜$nとNFが作り直される風に