verb--request-spec-from-babel-src-block calls verb--request-spec-from-hierarchy which evaluates post process function.
(defun verb--request-spec-from-babel-src-block (pos body)
...
(save-excursion
(let* ((metadata (verb--heading-properties verb--metadata-prefix))
(rs (verb-request-spec-from-string body metadata)))
...
(when (verb--up-heading)
;; Continue reading requests from the headings
;; hierarchy. Pre-include the one we read from the source block
;; at the end of the list.
(setq rs (verb--request-spec-from-hierarchy rs))) ; <- This calls `verb--request-spec-post-process`
(verb--request-spec-post-process rs)))) ;<- This is evaluated twice.
verb--request-spec-from-babel-src-block
callsverb--request-spec-from-hierarchy
which evaluates post process function.