BTCC cars getting faster...or not?

[ATF]

I noticed that the Brands Indy lap record is still held by Muller from 2001. I don't know if it's just me but I would have expected times to have progressed since them. I checked out the rest of the lap records (thanks to BTCC Racing) and came up with the following...

Brands Indy (2001-2005)
2001 - Yvan MULLER - Vauxhall Astra Coupe - Vauxhall Motorsport - 49.226

Brands GP (2002, 2005)
2005 - Rob COLLARD - MG ZS - West Surrey Racing - 1:33.148

Croft (2001-2005)
2002 - James THOMPSON - Vauxhall Astra Coupe - Vauxhall Motorsport - 1:25.749

Donington National (2003-2005)
2005 - Yvan MULLER - Vauxhall Astra Sport Hatch - VX Racing - 1:12.889

Donington GP (2001-2002)
2002 - James THOMPSON - Vauxhall Astra Coupe - Vauxhall Motorsport - 1:41.211

Knockhill (2001-2002, 2004-2005)
2004 - Colin TURKINGTON - MG ZS - West Surrey Racing - 54.845

Mondello (2001-2005)
2005 - Yvan MULLER - Vauxhall Astra Sport Hatch - VX Racing - 1:46.510

Oulton Island (2001-2005)
2002 - Matt NEAL - Vauxhall Astra Coupe - Egg Sport - 1:27.595

Silverstone International (2001-2004)
2002 - James Thompson - Vauxhall Astra Coupe - Vauxhall Motorsport - 1:28.324

Snetterton (2001-2005)
2005 - Matt NEAL - Honda Integra - Team Halfords - 1:12.249

Thruxton (2001-2005)
2002 - Yvan MULLER - Vauxhall Astra Coupe - Vauxhall Motorsport - 1:17.303

N.B - the Silverstone National and Rockingham lap records are irrelevant as they have only been used in 1 year (since 2001).

Looking at the results, they are quite inconsistent with no real patterns - you would expect the laptimes to decrease, otherwise in theory, Mullers 2001 Astra would still be competitive today?! Why is this the case?