This should be obvious: Average speed is not the same as maximum speed.
If I drive between my house and the University along the freeway, I'm reaching 80 mph (and traveling at that speed for a goodly portion of the distance). I think of my 'travel speed' as 80 mph, regardless of the time I spend waiting at lights, or driving cross-town to access the freeway. But when I use the distance traveled (~17 miles) and actual travel time (45 minutes) to calculate* my rate, I find out my average speed is much, much lower--about 23mph.
When transit planners talk about transit, they habitually talk about the AVERAGE speed, rather than the maximum speed, but fail to make a distinction between the two. For a bus, an AVERAGE speed of 8-10 mph is normal. 12 mph is really really good, while 4 mph sucks. A back of the envelope calculation on the AVERAGE speed on TRAX between Sandy, Utah and Downtown SLC is about 30 mph.
*Distance = Rate * Time, thus Time/Distance = Rate