The reason why there is only one GD16 glyph in the calendar of G is that the start of the calendar is the beginning of the 2nd quarter. In E the first vai glyph appears at the end of the 1st quarter, and then the G calendar has not yet begun:
E calendar:
1st quarter
1 2 3 4 5 6
2nd quarter
7 8 9 10 11 12
3rd quarter
13 14 15 16 17 18
4th quarter
19 20 21 22 23 24

According to E vai arrives in period 6 (marked red) after spring equinox (5) and disappears in period 15, before autumn equinox (18), i.e. vai is present during 10 periods.

According to G vai is implicitly present already in the 1st period and disappears with autumn equinox (17).