What speeds should Fibre give you?

Hi All / TT Mods,

Until the other day when browsed these forums, I kinda forgot about my speeds. When first had Fibre installed, I was getting speeds on average of around 20Mb. Whilst this was not the quoted 38Mb Fibre I paid for, it was a vast step up from the 2Mb standard Broadband I had... and just let it go.

However when reading other peoples posts, I decided to do a new speedtest on my line and was shocked at some of the readings. 

During the day, I get around 5Mb to 12Mb
During the night, I get around 21Mb

Neither of these are anywhere close to the 38Mb quote, I've expected and paying for....

Are these speeds justified, or is there something wrong with my line, gain ???

The router I am using is TalkTalks SuperRoute 635 I think it is...