You’re definitely on ADSL?
Sounds to me like you were on an EOL but at some point in the past Openreach have built a PCP/DSLAM outside the Exchange so essentially swapped you from a 3.9km EOL to a 50m E-Side and 3.89 D-Side.
FTTC uses a much higher frequency range than ADSL so the loss in Sync Speed is a lot greater than ADSL.