Mark
Good question.
The simple answer is that some tracks are built (surveyed):
1. to have a curb (Kerb for our friends on the other side of the Pond). The measured lines for these are 30cm from the outer edge of the curb.
2. not to have a curb. Measured 20 cm from the outer edge of the inside lane. Which is the same measurement technique as lanes 2+.
3. And then there are some tracks built(surveyed) 30cm out and never have a curb!.....which of course is not acceptable.
All outdoor international level tracks have a curb. It's required for World Records. Also, nearly all college/university tracks are built/surveyed for a curb.
Most high school & middle school tracks are built without a curb. Curbs are expensive and can be difficult to maintain.
There is a simple way to determine if a track was measured to have or curb (or not):
Measure the distance in lane 2 from the common start to the 1 or 2 turn stagger (it's easier to do the 1 turn stagger - also used for the 800 meter start). Remember to measure 20 cm from the left hand line.
Then measure the same in lane 3.
If the stagger is the same between lanes 1 & 2 and 2 & 3, the track was measured without plans for a curb.
If the staggers are different, then the track was measured for a curb.