Movement Speeds, Time Steps, and Map Scale

I've been looking at movement speeds across RPGs and noticed some huge differences.  How can something so well-known vary so much across representations?  Here some quick stats (cursory pull from core rules for first level characters, but representative).

Shadowrun
Round: 3 seconds
Walk: 8 to 15 m
Run: 20 to 35 m

DCC
Round: 10 seconds
Single Move: 20 to 40 feet
Double Move: 30 to 60 feet

Pathfinder
Round: 6 seconds
Walk: 20 to 30 feet
Run: 100 to 150 feet

AD&D
Segment: 6 seconds
Movement: 120 to 150 feet

Summary               MIN                       MAX
Shadowrun            6.0 mph                   26.0 mph
DCC                     1.4 mph                       4.0 mph
Pathfinder              2.3 mph                   17.0 mph
AD&D                  13.6 mph                 17.0 mph
Fastest 100m Olympics            27 mph
Typical human walking                3 mph
Racewalker                                 8.7 mph

Now of most interest is the maximum speeds.  Is DCC really that much slower?  I understand that the 26.0 mph is for trolls in Shadowrun, which is a somewhat unique race.  Also, each system has different ways of treating encumbrance.  Looking at the whole collectiion, besides DCC numbers seeming a little low (which could be my misinterpretation), I think the ranges look reasonable.

The impact of these numbers -- namely the time step and max movement speeds -- is the reasonableness of scale to use in mapping..  In Pathfinder, 5' squares seems really comfy, but I have a feeling in Shadowrun, 5 meter squares will probably be easier in environments where players are going to be running around a lot.