Hm.. the number comes from the BibTeX field "volume", which should AFAIK be used for multi-volume book (e.g. "Algebra : Volume 1"), but not for a book in a serie with numbers. A book in a book serie has an individual title and if it is numbered, one should use the BibTeX field "number". Right?

This would mean that the BibTeX from the following providers is wrong w.r.t. to volume/number:
http://www.ssoar.info/ssoar/handle/document/1919?style=bibtex
http://dblp.uni-trier.de/rec/bibtex/conf/approx/2013
http://link.springer.com/export-citation/chapter/10.1007/978-94-011-3466-8_4.bib