I've encountered this problem too. It's apparent in the following
query, which returns me an empty posts array but a total_posts value
(282) that would imply that my offset (230) is fine.
curl '
http://api.tumblr.com/v2/blog/iwdrm.tumblr.com/posts/photo?
api_key=xxxx&limit=1&offset=230'
{"meta":{...},"response":{"blog":{...},"posts":[],"total_posts":
282}}
FWIW, I ran some JS against the archive page of that blog and it looks
like the total_posts number is the inaccurate one - the limit I'm
finding experimentally is the correct number of posts.
Ian