There is more than one way you can measure a garment. A good spec chart will note not only the placement of each point of measure on the garment, but the method by which it is measured.
There are four basic methods: along the X axis, along the Y axis, straight, and along the contour.
Measuring along the X axis is measuring straight across the garment from left to right at a certain point like the bustline, sweep, or leg opening.
Measuring along the Y axis is measuring straight up and down the garment for vertical measurements like length or neckline depth.
Measuring straight does not follow the curve of the garment nor does it follow the X or Y axis. Instead the measuring tape is laid straight from one point to another.
Measuring along a contour follows the curve of the garment along a seam or edge such as an armhole, hem, or princess seam.
The point of measure, style of the garment, and even fabric influences which of these methods is the best to control each measurement.
I wrote more about measuring methods and understanding points of measure in a blog article here.