|Format F14 as a Number. Excel is showing a Custom Format of mmmm|
It appears that only F14 is formatted as mmmm. F15:F19 are all formatted as General, which should return the actual average.
Based on the values used in the spreadsheet that you posted at the Ozgrid site, the average for January is 369.4.
369.4 is Excel's serial number for January 3, 1901 9:36AM. Excel is picking it up as a Date because of the Custom Format of mmmm and returning January.
BTW...Since you are using Excel 2010, you should use AVERAGEIF (after reformatting F14) since array formulas can be cumbersome to edit and use more resources.
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