Altium trace length vs signal length
If you have access to Altium's forum please look at these two threads:
https://forum.live.altium.com/#posts/215294
https://forum.live.altium.com/#posts/215784

If you have no access, the basics are:

Routed length - sum of all segments of a net. Having overlapping segments will give falsely larger numbers.

Signal length - calculated as length of path along a net between two pads. Signal length = 0 means there are either 1 or 3+ pads in the net. Altium can only calculate signal length for a 2-pad net.