Attenuation is dependent of frequency.
For example, the following is the Attenuation vs. Frequency for an RG-8 (coaxial) cable:
At 1Mhz the Attenuation is 0.1 dB for each 100 Feet.
At 100 Mhz is 1 dB for each 100 Feet
At 1 Ghz is around 5 dB for each 100 feet.
dB is a logarithmic measure, so these differences are huge.
Your Downstream speed (64286 kbps) uses signals in higher frequencies, maybe hundreds of Mhz.
Your Upstream speed (18782 kbps) maybe is located below 10 Mhz, so the attenuation is higher for downstream.
Here is other graphic involving multiple types of cable: