I assume mr_mojo is right, but would add this is why we sometimes recommend people on longish lines on ADSL2+ to force it down to ADSL2.
The logic is that some router firmwares seem to calculate a higher attenuation on ADSL2+, (approximately 3dB), perhaps by taking into account the attenuation averaged over all the theoretically available frequencies on ADSL2+, but others seem somehow to leave it unchanged from the ADSL value.
Then they report back the attenuation to the DSLAM during the sync negotiation and together with the noise values end up with an agreed sync. That's my theory anyway.
Because the attenuation is higher, but the expected additional frequencies are in fact unusable, a lower sync is obtained than if the connection is forced down to ADSL2. As ADSL2 uses the ADSL frequenies as already covered, the lower attenuation is regained, and the sync speed can increase.
Where the crossover point between ADSL2 and 2+ comes varies from line to line. A very small percentage even end up best on ADSL, for some reason I haven't thought of.
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