I have noticed a difference when I (1) average seasonal data by creating a climatological axis with 3 month-long cells then @AVG regridding, verses (2) when I separate each month into its own variable and then average them together using expressions. It appears that the latter method is doing what I expect,
But, the length of the month varies and therefore (1) should be the correct average: For example,
where J, F, M are the monthly means of Jan, Feb, and March, respectively. The @AVG averaging takes the monthly variable lengths into account. This principle applies to other axes with uneven grid-spacing, too.
Hope this helps,
Ryo