Because of optimizations, most likely. There are a total of 16 384 columns and 1 048 576 rows in Excel 2007. Weird figures? 20 bits = 1 048 576, 14 bits = 16 384
More than one 32-bit register can hold when 14 + 20 = 34 bits are added together.
However, they must also save the cell's format (text, number, etc.) and formatting (colors, borders etc). They use 34 bits for the cell number and 30 bits for other items if they use two 32-bit words (64 bit). What is the significance of this? They don't need to allocate all of the memory required for the entire spreadsheet in memory; instead, they just need to allocate the memory required for your data, and each piece of data is tagged with the cell it should be in.