The RG13, piece=3167, folio=85, page=19 reference reflects the archival arrangement of the census collections at The National Archives, so is the well established convention. However, it is based on microfilm records and Ancestry's records are digital images.
The original Census Enumerators Books (CEBs) were arranged by 'registration/adminstration county', registration districts, sub-districts and enumeration districts. Consequently a TNA piece collected together a number of CEB's from an area. With more than one book per piece page numbers are not unique. When the censuses where microfilmed, the folio numbers were added (stamped on the top right of the front of each sheet of paper in the book), which combined with the page number ensures a unique reference for each page.
Ancestry appears to follow the original CEB arrangement, but substitutes an image number instead of the page number. As the first page in a CEB is not numbered ( it has details of the enumeration district, including the enumeration district number), the image number should in theory be one more than the page number (e.g. page number 19 equates to image number 20). Individual census pages can be accessed using the 'Browse this collection' function.
When citing an online record, I suggest that the website's arrangement is included for ease of retrieval, as well as the archive reference. In this case, I suggest:
Census 1901. England, Nottinghamshire, Nottingham, Nottingham South West, District 29, image no 20. Digital image. Digital image. Ancestry (www.ancestry.co.uk : accessed 21 November 2012); citing The National Archives, Kew, RG 13/3167/85/19.
Christian, Peter & Annal, David. (2008) Census: The Expert Guide. The National Archives, Kew. pp.252-253, 48.