What does "reaction rank" in Reaxys stand for?
For the Elsevier's Reaxys data base, there is a support page you may consult without subscription, nor log-in to the product.  Actually, this is more like a searchable directory, too, because you search for keywords, like «rank», too.

Doing so here -- time stamped 25/04/2019 -- offers insight about «How do I view my results?».  About half-way through this document, in subsection «Reactions. Order, limit or export your search results», the documentation states:


  "[Reaxys ranking] Defines the method of determining 'relevance'. To
  change the sort type, click 'Reaxys Ranking' and define another sort
  option: 'No of References', 'Reactant Availability', ‘Product
  Availability'', ‘MW of product’, ‘Yield’ or ‘Publication Year’."


And because the content of columns of numeric content (like molar mass, number of reagents, year of publication) may be sorted, this may affect the presentation of the export, too.  It equally may explain the occurrence of gaps in the values you mention.