Since the charging system wasn't designed to charge a car battery, I'd say it's very possible that the huge battery is the reason for the reading you are getting.
The stator output max may be 14-15v, but with a huge battery in the system pulling all that voltage to charge you are essentially getting a battery voltage reading from your tester..
Disconnect the battery and repeat the test with engine running..without the load of the battery you should see the spec the book says.
And for god sakes man..get a proper battery!!
Current toys..
1986 Honda 350X..trail bomb!
1985 Honda 250SX..my main mudder
1985 Honda 250ES..Back in Black Trike
Current non-trike toys:
1990 Honda TRX300FW
1995 Seadoo GTX
1998 Polaris Indy Lite 340(Nearly new looking)
1998 Polaris Touring 500
1998 Club Car (electric)