I beleive it is because there is a limit as to how many rows you can run an array formula on.

I have not used array formulas for years. When I did use them I remember running into similar problems once the number of rows got up around 1,000.

Have a look at
http://www.ozgrid.com/Excel/arrays.htm
http://www.ozgrid.com/excel-add-ins/array-formulas.htm