If you start using additional higher frequencies then this could change the weighting used by the modems in calculating the attenuation figure. High frequencies are attenuated more than lower ones. The calculation is some kind of average-like process combining the different figures obtained at all the various frequencies into one single result figure, so this combination calculation provides a likely answer.
A less likely possibility sometimes is that something physical about a line has changed, such as the use of thicker or thinner copper, or that a modem or its firmware has changed so that the analog front end is different, or perhaps the attenuation calculation software is different following a change of modem or a firmware update.
When some of my customers changed from BTW 0.5 Mbps ADSL to approx 7 Mbps downstream ADSL "Max" service back in 2006, their reported attenuation figures changed a lot. This was due to the calculation including additional, previously unused higher tones.