You should do the math again.
There are 29.5306 days between two full moons. That makes 369.1325 days in a year, if you calculate 12.5 moons.
In 1 year, there are 365.242 days
In 9 years, there are 3287.178 days.
In 27 years, there are 9861.534 days
If you take 12.5 moons for a year, you have 112.5 moons or 3322.1925 days in a year.
If you take a moon out of the last month of the ninth year, you have 111.5 moons or 3292.6619 days
If you take 1.5 moon out of the 27th year, you have 334 moons in 27 years. This is 9863.2204 days. Not 100% accurate, but close.
In the solar-wagon (found at Trundholm, Danmark), there is a shield with four concentric circles. The three outer circles have 8, 16 and 27 circles. The inner circle with the second circle with the eight circles together form the year of nine periods: 8 months and the "time between the years". The outer circle with the 27 circles points to the 27-year period.
Once in every 9 years the tribes send representatives to Uppsala (Sweden) to negotiate the time for the next nine years. In this period, the "peace-of-Freyr" made sure, that there were no hostilities against any representative.
The numbers above are from the website of Andreas Firewolf.
http://www.andreas333.com/nl/yggdrasil/boek/ygg1_h4.htm