This chapter is where the Gita's manuscript history becomes visible to an ordinary reader, because printed editions disagree about how many verses it has, and the disagreement shifts every verse number in the chapter by one.
At issue is a single opening verse in which Arjuna asks about nature and spirit, the field and the knower of the field, and knowledge and what is to be known. The recension followed by Shankara does not include it, and gives the chapter thirty-four verses beginning with Krishna's statement that the body is called the field. The recension followed by Ramanuja includes it, and gives thirty-five. Editions in circulation today follow one or the other, frequently without saying which, so a citation of chapter thirteen verse twenty may point at two different verses depending on the book in the reader's hand.
The atlas records the verse count for this chapter as disputed rather than choosing a number, and the same caution applies to the traditional total of seven hundred verses for the whole Gita. That figure holds on the shorter reckoning of chapter thirteen. It is also worth knowing that a longer recension of the Gita survives from Kashmir, with additional verses elsewhere in the text, so the seven hundred is the count of the widely printed vulgate rather than of every witness.
None of this is a scandal, and none of it is hidden. It is the ordinary condition of a text transmitted by hand for two thousand years inside a much larger epic that was itself transmitted in several forms. What it does mean is that a reader comparing two translations and finding the verse numbers out of step has not made a mistake.