The old saying goes that there are 'lies, damned lies, and statistics'. This is true in many areas, but especially when calculating the length of a trail. Estimats of the base mileage of a trail vary wildly - for instance some say the Pennine Way has a length of 265 miles, whilst others put the length as high as 270 miles.
For this reason, all mileage distance should be seen as being approximate. The figures on this website are taken from GPS logs that have been cleaned on a computer-based mapping program. As such they give a fairly accurate measure of the distance that I walked on the trails. Others will get different values depending on how they measure the trail (and, indeed, which loops and paths actually constitutes the trail).
Likewise, many trails have routes that split and rejoin later, for instance the Bowes Loop on the Pennine Way, or the high-level route to Kirk Yetholm on the same route. In these cases I have arbitrarily picked a 'main' route and discuss the branches as alternatives.
Ascent and descent figures are much more varied. They are generated from the computer mapping program, and for various reasons can give wildly exagerated values, especially on clifftops or in steep-sided valleys. For this reason it is best to treat them as a guideline rather than as exact values.