Wing aspect ratio vs. airspeed ?

Today I did some volunteer work, screening potential entries into a
high school science fair. Lots of interesting experiments, but one in
particular caught my eye:

The student had created an RC plane out of styrofoam, balsa wood and an
electrical pusher prop. Now the experiment consisted of clocking the
time it takes for said plane to cover a distance of 100 yards, but with
two different sets of wings. Both wing sets are planar triangles (no
camber) and have the same surface area. One however had a wider
wingspan than the other. Thus one configuration looked more like a
delta wing than the other.

Interestingly enough the result was that the delta wing with shorter
wingspan travelled more slowly than the other wing. I thought D-wings
are generally less stable but faster, no !?