There may be an advantage to having the downspout slightly out of vertical.
I deliberately added some slope to one of my downspouts in my old house
because it made a loud dripping moise anytime there was even the slightest
rain. It was loud and it lasted for hours and it was right beside my
bedroom window. A bit of slope helped the water run along the walls of the
downspout rather than freefall. The noise stopped.

Perhaps your contractor used a small amount of slope for a similar reason.
I think it may be good practice to not install downstops perfectly vertical
because it avoids noisy dripping.