The BT checker show the likely synch speed, between the fibre cab and your router.
The BT checker does not show the download speeds / throughput capability of the line or circuit [to use the engineering term].
The throughput will always be a bit less that synch speed due to what is often described as overheads.
For detailed explanation please see https://kitz.co.uk/adsl/IPprofile.htm
Re: My Fibre Extra download seems a little slower that it should be
Thanks for getting in touch and welcome to the forums.
I have looked into your connection and can see the speed is spot on from our side and is within your estimates. The information provided by @RichardB is correct and will like the reason for the speed you are seeing.
However I have noticed there has been a few drops today which may very well be nothing, however I ask that you get back in touch if these persists or increase.