Electrical issues - Alternator? Rectifier?

[Dead Eye]

Just a quick query; from doing some electrical work on my Gen 1 the other week, my battery was registering at around 15v even though the bike had been sat off for a several hours - I'd suggest my multi-metre is out since it is old (and analogue) but it reports correctly on other batteries (think AA or 9v batteries)

So, ignoring the multi-metre, any reason for the high voltage?