Attainable v's Actual

[#1]

Recently moved from ADSL1 to Fibre. First couple of days router was obviously trying to find the best possible speed so line dropping every few hours or so.
Since then - all is stable but. The Billion router suggests Attainable Rate of 12000Kbps or thereabouts whereas the actual is showing as 9875Kbps. The SNR has been set on the router as 6 which seemed to be a good figure to go for with ADSL1. Attentuation is 33.6.
Now I know there are other factors that come into play especially with my antiquated copper wire from the cabinet to my house and the BTOR engineer did suggest that they could install a gismo (apologies- I forget exactly what she said it was called) at my house to increase the line speed. I believe it increases the line voltage but I could be wrong.
Any ideas anyone as to why the Attainable seems so far away from the Actual. Also, in the absence of being able to contact BTOR direct - what exactly was the BTOR engineer talking about.
All input appreciated - attached screenshot of Billion stats  .

[#2]

I have a similar result and I suspect it is the effect of interleaving being applied as that dropped my speed from about 48 (reported by BT Engineer when it was installed) to 43.9

[#3]

Thanks for the feedback. When it was first installed with the BT modem and PN router it was showing 4000Kbps. The engineer suggested that the Billion probably would be better and hey presto, the speed suddenly jumped to to over 10. Getting a fivefold increase over my old ADSL1 with an attenuation of about 63 was a great improvement.
PN suggested it should be in excess of 11 but probably too early to raise a ticket just to 'grab' that extra 1Kbps. Wish I could remember exactly what that fine tuning that BTOR could do my end to improve things.
Maybe PN technical support will add some clarity to my confusion.