Orders API doesn't return all data.

Shopify Partner
15 0 1

I am trying to extract all Orders and my app has read_all_orders permission too. 

We generally use a timestamp to filter our queries. the filter criteria is like updated_at_min >= xyz and update_at_min < abc timestamp value. 

First we get a count say C1, 

During extraction we get a different count (C2) (with C2 being less than C1). This is expected because some records might have been modified. 

After extraction we again do a count with same query parameters. (C3). Now C3 is greater than C2. 

This is somewhat surprising.