Hi Charles,
this is a default behavior but you can disable it by configuring the "Font replacement table" inside the PDFCreator printers properties->device settings
Helvetica is one of the Fonts defined in the PDF standard, while Arial needs to be either installed on the target device or embedded into the PDF (which PDFCreator will do, but it increases the file size).
Since they have the same metrics and there is usually no reason to not use Helvetica instead of Arial, this replacement is enabled by default.
(Sorry I am currently sitting in front of a German Windows)
Best regards
Robin