Because allowance needs to be made for Layer 2 overheads.
The line rate (sync speed) is a Layer 2 bit rate. The profile is a Layer 3 IP rate.
IP packets are encapsulated within a Layer 2 frame/cell before being sent to line, with the extra data needed being the 'overhead'.

A 12% (ish) overhead is assumed for typical traffic.

Search for 'OSI model' for more information.

Also interesting that since TR101 first appeared under "BRAS Profile" for my "Line Rate Data" in the Portal on 15 December, a new row of data has appeared in the morning of alternate days - even though the numbers themselves are the same each time. So it seems that, whereas previously there was only a new entry whenever something changed, now there's a new entry every other day.

p