Paging, using after link

[PALY]

Hello,

Using the API, i receive the following header :

<link type="application/atom+xml" rel="next" href="http://
api.collecta.com/search?page=2&amp;q=adidas" />
<link type="application/atom+xml" rel="self" href="http://
api.collecta.com/search?
page=1&amp;q=adidas&amp;since_id=1288823825849" />
<link type="application/atom+xml" rel="refresh" href="http://
api.collecta.com/search?q=adidas&amp;since_id=1288823825849" />
<link type="application/atom+xml" rel="after" href="http://
api.collecta.com/search?q=adidas&amp;after_id=1288749055646" />
<opensearch:totalResults>70</opensearch:totalResults>
<opensearch:itemsPerPage>15</opensearch:itemsPerPage>

I understand it as : there are 70 total results, here are the first
15.

Using the after link, i think it would remain 70 - 15 = 55 items, but
no : querying the after URL says it remains 50 !
5 are lost.

Anything i am misunderstanding ?

Thanks for feedback,