As well as 'we probably get more rain than you (due to topography?)', the apparent rainy bias might also have been influenced by the fact that I just discovered that my plaintext-parser-data.txt was an old copy (the copy that came with the rest of the php files, rather than the latest one specifically from the page about using the WXSIM php forecast).
I discovered that because it seems odd that a 10% chance of rain was being described as "Rain likely"! Hunting for the text "likely" led me to the answer...
Updating that file (and plaintext-parser.php, but the difference there was trivial), has changed "Rain likely" to "Chance rain". The newer plaintext-parser-data.txt also has an extra line in it and suddenly the forecast icons on my page show a lot less of the 'cars in rain on a (slight incongruous around here) multi-lane highway'.

Maybe there was an 'off by one' problem without that extra line?