The reasons for re-scaling are dependent on some decisions made during the development of WN8.
You can see the technical side and logic for this in the Nuts & Bolts section.
Also, it is very easy to rack up kills in a tier one tank, skewing KPG upward.
WN8 should not be considered the be-all-end-all of skill evaluation.
It is intended to be looked at next to win-rate, battle count, average tier, and will never replace the inspection of the Service Record or platooning with someone or watching their replays.
Also, damage dealing capabilities are hardly balanced across all tanks of the same tier (think T40 vs A-20 for example or the ARL V39 and M18).
Kill per tier scaling – lower skilled players are found in lower tiers, and thus getting kills at tier 1 is easier than getting kills at tier 10, holding constant for player ability above a certain threshold.
This is because WN8 cannot capture things like timing, target priority, teamwork or decision making abilities.
As with all ratings before it WN7 had some weaknesses and limitations.
This phenomena is perfectly illustrated by tier 1 autocannons, which can clip out opposing tier ones before the new players even can turn their turrets and react.
This sort of destructive potential allows players with relatively low skill to win an extraordinary portion of low tier matches and inflate their stats.
Some of these were known at the time of release of WN7 and others were discovered and publicized shortly thereafter.
In order of seriousness, as judged by the WN* team: Damage per tier scaling – namely that while tier is a nice linear 1-10, damage values scale up in a curvilinear fashion.
, Damage Dealt, Damage Received, Kills, XP, Detected, Capture Points, Defense Points, WN7): If you were to average the individual WN7 values of these games, you would arrive at 2578.