There are several reasons why a query that performs large I/O might require more reads than you anticipate:
The cache used by the query has a 2K cache and other processes have brought pages from the table into the 2K cache.
If Adaptive Server finds that one of the pages it would read using 16K I/Os already in the 2K cache, it performs 2K I/O on the other pages in the extent that are required by the query.
The first extent on each allocation unit stores the allocation page, so if a query needs to access all the pages on the extent, it must perform 2K I/O on the 7 pages that share the extent with the allocation page.
The other 31 extents can be read using 16K I/O. So, the minimum number of reads for an entire allocation unit is always 38, not 32.
In nonclustered indexes and clustered indexes on data-only-locked tables, an extent may store both leaf-level pages and pages from higher levels of the index. 2K I/O is performed on the higher levels of indexes, and for leaf-level pages when few pages are needed by a query.
When a covering leaf-level scan performs 16K I/O, it is likely that some of the pages from some extents will be in the 2K cache. The rest of the pages in the extent will be read using 2K I/O.
