It seemed to be fairly common when maxdsl first came in, but gradually over the next few years the higher sync rate appeared more often.
This could be due to updating of the exchange MSANs (or MSAN firmware?)... also more people would be updating to newer routers.
When maxdsl first came in, many users were still using the same routers that they had for say 512/1Mb/2Mb fixed rate. Whilst many of these would work fine with maxdsl, not all of them (eg the old Solwise/Voyagers some of the older STs - even the old frog modems were still in use) supplied by some ISPs back then were not adsl 2/2+ compatible nor able to use s=1/2 mode algorithm.