I see this effect too, on a regular daily'ish basis. Downstream SNRM leaps from 6 to 10 or 11, the downstream max rate leaps from the usual 39 mbps to circa 45 mbps, then at the next 15-sec point, drops back to normal.
My Plusnet service is the 40 mbps, but for the first 6 months was running at around 45 mbps max rate (the Plusnet profile ran at circa 39 mbps), then a month ago the max rate also dropped back to below 40 mbps. I had concluded it was probably the DLM trying to put the line to max possible achievable speed, but then some process chopping it back to the 40 mbps service max.
But I used to be on ADSL with 2 mbps (max) so it is more of a matter of interest than concern.