2 reasons - those 5 are overall distinctively better squad than rest 5. And, they play lots of game among themselves (and don't play many against lower tier).
First point is probably self explanatory. 2nd point is maths - unless you are playing higher ranked teams, one can't make up point (or loose points to drop down). Those 5 are playing lots of games among themselves and regardless of results (unless it's a crushing), neither team makes much (or lose much). For example - even a 4-1 (can be either side) doesn't change SAF/IND's overall point much - it's 1 vs 2. Problem for bottom half teams is that, they don't play much against top teams - so, 1. if they sweeps a lower ranked team, still can't make grounds, 2. the top half teams don't lose enough to drop down that much.
For example - we made up 29 (?) points in 9 months from 58 (?) in Nov '14 to Sep '15 from 5-0 (ZIM), WC QF (win against Poms & not losing against AFG, SCT); 3-0 (PAK), 2-1 each (IND/SAF), because the base was 58. Similarly, PAK made up almost 10 points from 3 wins against IND, ENG & SAF (& not losing against SRL) at CT. On contrary, winning/losing 5-0 against SRL/NZ hardly had any impact (2/3 points I believe). Had some how this result been 0-5 & 5-0 in opposite direction, still points would have been same for the surprise loss against SRL & surprise win against NZ. PAK had a chance to cover 9 points I believe had this one been 5-0 for PAK..........
Ranking system is done in a way that, playing lots of games within same level hardly helps - PAK-BD can play 3 series at home, away & neutral for 8 X 3 = 24 games; if W/L remains close around 12/12, hardly their is a point change. Similarly, IND-AUS can play 7 match series every year and their ranking would be safe at similar spot - W/L doesn't earn or cost much there, unless it's a sweep.