Simple formula returning odd result

Hi all

I have a reasonable grasp of Excel and this has me completely stumped.

Three cells hold numeric values (format General though the format seems not to change the result)

C4 = 4082322.25
D4 = 4046689.05
E4 = 35633.2

Another cell contains the simplest of formulae ...
F4 = "=C4-D4-E4"

Clearly the result should be zero.
In fact the result is (to 15 decimals) ...

0.000000000189175

HOWEVER
If I change F4 formula to "=C4-E4-D4" the result is zero.

Any ideas