Not sure this is really an answer but...
The 6 daye baptized ?Elizabethe? & Annis daughters of William
If you're in Ancestry and switch on the index pane (two heads and = to the right of the number), you can see Annis Lorberg and the surrounding index entries. Thomas Green is above, John Harrison below (modernising my spellings). So you're looking in the right place.
What's also apparent is that her sister, Elizabeth, is indexed immediately above Annis but with the surname of Leyland. So our invisible surname is written down two different ways!
The only possible explanation I have is that the index is not an index of the images seen but is an index compiled at a different time. If you look in the FamilySearch catalogue on this Lambeth entry, we find something labelled
Transcripts of parish registers of Lambeth, St. Mary, Surrey, England,
Statement of Responsibility: compiled by W. H. Challen
My only guess is that (a) WHC did his index from the real thing so could see into the gutter between the pages (it's the gutter / fold between 2 opposite pages, it's not been cut off, I believe) and (b) Ancestry copied his index and linked it to their images. The flaw in my suggestion is that it doesn't explain why the 2 surnames are different - one might have been corrected by a user but I struggle to see evidence of a correction.
NB - WHC's index appears to have been digitised but can only be seen at an LDS Family History Centre or at an Affiliate Library,e.g. the Society of Genealogists in London.