What version of XL are you using?

=ROUNDDOWN(((7-1)/3),0)

returns 2 in Mac XL04.

If instead of 7, 1 and 3, you're using references to cells with
formulae, it's possible that there are rounding errors in your formulae
that are causing the ROUNDDOWN() to round to 1.

For instance, if the cell displaying 7 instead contains a formula
returning

6.999999999999993

due to rounding errors, the ROUNDDOWN will operate on the stored value,
rather than the displayed value.

In article ,
"M8" wrote:

I can't get ROUNDDOWN to work with embedded operations.
For instance:
=ROUNDDOWN(7-1,0) returns 6
BUT
=ROUNDDOWN(((7-1)/3),0) will return 1 instead of 2

what is going on?