What determines whether strings are saved as wide UTF-16 or ASCII?

I have a rather obscure question about how AppleScript encodes strings when saved. I have two seemingly similar scripts that use “do shell script” and then execute curl with a supplied URL.

In one script, the strings were saved as wide strings; in the other, as plain ASCII. There are no differences in the scripts in terms of specifying encoding. Aside from using different URLs and a few user defined terms, the script source files are more or less identical.

Might it depend on the tool used to create/save the script, or the existence of some particular character/characters used? My hunt for any documentation on encoding when no encoding is specified drew something of a blank.

TIA

It’s not documented anywhere, but I believe it saves as ASCII of the characters used are in the 128 ASCII set, otherwise it uses UTF (UTF-16?).