This is pure speculation and not anything proven, but I sometimes wonder if things like this may have something to do with the kit at your particular exchange.
Now Im not sure if this will apply to max - but the theory is still there.
But "back in the old days" of fixed adsl when speeds were a lot more static and you could spot certain things easier, but there were definately some exchanges which seemed to allow "a bit extra" to cover atm overheads, and this affected your max throughput speed.
Chris noticed this when he moved to a neighbouring exchange and it affected the maximum throughput speed no matter how he tweaked etc with the same kit and router. When we were both on the same exchange despite being on different ISPs at one point - the maximum throughput speed was always the same within 1 or 2kbps
Something else I noticed when on 2Mb during summer 2005 and running up to max there were times when I could very often get speeds of up to
1900 on the AG speedtester with 1700kbps being quite common.
These speeds were supposed to be impossible for a 2272 sync but it happened way too often for it to be a fluke. I also used to see other people on different exchanges reporting the same thing - and there was definitely a period when quite a few users were reporting more than the theoretical max throuput of 1920 kbps that should be possible on a 2272 sync.
It would therefore seem, that no matter what the rules should be.. there sometimes is some exceptions to these so called BT rules.... or I could be talking just pure rubbish.
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Edited to add
Despite what BTw "say" it is possible to attain a sync of more than 7616 kbps on an interleaved line.
This is dependant on both the DSLAM and your router being able to support S=1/2 mode which effectively combines two RS code words into a larger logical code word of 510 bytes (ANSI T1.413), and there being sufficient spare SNRM.