Bends in the fibre can't cause that. It will work or it won't.
You're getting 2.4Gb to the ONT from your fibre, it's just rate limited to 1000Mb.
The fibre can definitely handle 940Mb.
Some are only getting 910Mb.
BT Wholesale might be using different settings on different kit or in different areas.
If you're consistently getting 920Mb max then that's how your line has been setup somewhere.
For others it's only 910Mb.
I doubt you'll ever speak to anyone who can tell you why the differences between users but as it's sold as a 900Mb package there will be very little point in raising it with BT.
Different sites can report throughput in different ways also.
Have you tried fast.com?