They're not grabbing the top 5% of all users, but taking an 'average' of all users.
AT&T's Uverse internet starts out with 3Mbps/1Mbps and charges extra. If they add in the ADSL users, many never make it past 3Mbps (2.4Mbps after ATM overhead).
TWC (and most cable providers) first bundle in +10Mbps/1Mbps (they'll charge more for a 1.5Mbps if you wanted to downgrade in a bundle!), 'then' they add on 'Boost' to fluff the numbers with a 20-30Mbps download rate for the first 10-20 seconds.
Eg. My '10/1Mbps package'
said by pcmag :
Who scored the lowest? Folks in Alabama, South Carolina and Mississippi appear to stumble thanks to AT&T South (formerly Bellsouth). All had an index of well below 5.0 (Alabama's AT&T speed is the lowest on the chart, with an index of 2.56.). The District of Columbia comes close, with Verizon getting an index score of 3.19 (despite the fact that FiOS is available in D.C.).