Well, I've been into it a bit more and wondered if the burner was reading the ATIP information on your CD-R and adjusting the speed reported to Nero according to what it finds either in the ATIP entry or by performing a pre-burn test. Ricoh very kindly address this point at this URL:
####://www.ricohpmmc.com/e_europe/drive/mp7163a/index.html

I think that's what's happened.