CaTGC

Lol, sounds like a pretty shitty ISP. Meaning they don't do documentation properly.

If you have an option to change the provider then you can always consider that.

I assume you're promised 100Mbps down and 10Mbps up? I'd also like to think you're having copper technology due to asymmetric speeds coming from VDSL (very-high-bit-rate digital subscriber line). Do you live in the middle of nowhere?

A. Attenuation on the copper line is bad (there's probably many repeaters on the line that amplify the signal, but in the end it might result in loss of speeds (this is if you're getting your connection straight from the DSL).

B. If you live in an apartment building then you're most likely using some copper-hybrid-optic solution. If so, then there is a network switch somewhere in the house that splits connections between customers and it has an uplink of 1000Mbps - optical cable that runs to MC (media converter) and into the switch (SDF or ODF is also possible).
Switches usually have 20 ports that are assigned for customers and for copper-hybrid-optics technology they're usually 100Mbps each. If the 1000Mbps uplink is overloaded by customers behind the same switch then some customers behind the switch might have some issues with speeds.

C. The VDSL card or switch is having high BER (bit error rate) and ISP never cared to fix it/check it.

D. ISP has made a mistake assigning your speeds. We make mistakes too.

I don't know, man... depends - I need more details.

Omlette du fromage

Marcus, accepted!