Yes, it's normal, or at least common. On ADSLx mine used to vary by 3.5dB over 24 hours. Nothing to worry about.
That's why the standard margin used to be set to a minimum 6dB, to cater for greater variation.
It also depends how good the router is at coping - Speedtouch 585s I found typically lost sync when he margin fell below 3dB, whereas NetGear 834 series would hold to 0dB or even slightly negative values.
My broadband basic info/help site - www.robertos.me.uk
| Domains,website and mail hosting - Tsohost
Connection - Plusnet
UnLim Fibre (FTTC). Sync ~ 59.4/14.4Mbps @ 600m. - BQM
"Where talent is a dwarf, self-esteem is a giant."
- Jean-Antoine Petit-Senn.
Allergy information: This post was manufactured in an environment where nuts are present. It may include traces of understatement, litotes and humour.