Excel calculation problem

I think something in Excel 2007 is trying to be too clever by half (or is it
a bug?).

Col A Formula Result Expected
2.25 =a$1 2.25 2.25
2.25 =a$1 + a2 4.50 4.50
3.50 =SUM(a$1:a3) 8.00 8.00
1.75 =SUM(a$1:a4) 8.00 9.75
2.00 =SUM(a$1:a5) 8.00 11.75
5.25 =SUM(a$1:a6) 8.00 17.00
9.00 =SUM(a$1:a7) 8.00 26.00
8.25 =SUM(a$1:a8) 8.00 34.25

The formula in cells B4:B8 was copied from B3. While inspection of the
formula indicates the correct change of row (as shown above), the calculation
doesn't reflect it (it acts as though the formula is =SUM(a$1:a$3). The
calculation is correct if I enter the formula specifically into each cell,
but my real situation calls for hundreds of cells, so I want the copy to
work. Thanks for any help.