Why are the runways aligned exactly to the runway number ie runway 24 heading 240. At Heathrow, the runways are 5 degrees off (so runway 27L is actually heading 275), but the white runway markers are aligned 270. I have found this at all the airports where the runway is not exactly aligned with its corresponding number.

Are you talking FS2004?FS2004 used data from about October 2002 for runways and such.Rwy 09L/27R at EGLL is at 89.690/ 269.690 degrees true - 92.7/ 272.7 magnetic.Rwy 09R/27L at EGLL is at 89.700/ 269.70 degrees true - 92.7/ 272.7 magnetic.Newer airport information data would certainly have the runways at different headings due to changes in the magnetic variation.

According to the latest UK AIP for Heathrow (18 Jan 07):RWY 09L is 089.67* TrueRWY 27L is 269.71* TrueRWY 09R is 089.68* TrueRWY 27L is 269.72* TrueMag Var is W2.3* (2006) - 0.14* decreasing