Newbie question re. sharpness

Hi,
I have what is hopefully a pretty basic question. I have a Canon 350D
with the two kit lenses:

* EF-S 18-55mm 1:3.5-5.6 II
* EF 75-300mm 1:4-5.6 III USM

As a test, I tried taking the "same" photo with each lens; that is, I
framed a still object as closely as possible with each lens and used
the same settings: Manual mode, 1/100s, f/5.6, ISO-100, Flash
(Speedlite 430EX pointed ahead), tripod. Obviously I had to move the
camera back to get the same shot with the 75-300. The only other
difference was of course the focal lengths: 45mm vs 75mm.

The picture taken with the 75-300 came out much sharper than with the
18-55. What accounts for that? I ask because, in general, I'm able to
get better looking pictures out of the 75-300.

Thanks in advance!