Learn something new everyday - why one blank cell causes a count in pivot tables (Excel)

Picked this up from Pivot Table Data Crunching Microsoft Excel 2010 (pg. 63):

If all cells in a column contain numeric data, Excel chooses to sum.  If just one cells is either blank or contains text, Excel chooses to count.

I have definitely encountered this before when building pivot tables, and just shrugged it off a quirk of Excel.  Interesting! Now I know why Excel Pivot Tables would default to a Count of Values instead of Sum.  Next time I'll ensure my base data doesn't contain any blanks. 

Any other pivot table tips out there waiting to be discovered?