Clustered index data access
This is explained in The Read Ahead that doesn’t count as Read Ahead


  what I wanted to talk about in this post, was another prefetching
  mechanism that can be triggered by any query when SQL is running any
  of the editions considered Enterprise (i.e. Developer, Evaluation, and
  Enterprise itself). The aim of this optimization is to warm up the
  cache as quickly as possible. To do so, the buffer pool converts any
  request to read a single page from disk into a request that will read
  the whole extent containing the page initially requested.


In your case you ran this after clearing the buffer cache with DBCC DROPCLEANBUFFERS. So SQL Server is in the state that it would try and warm up the cache quickly.

You needed to read three pages to perform the seek (one at each level of the index). These were pages 1111, 1382 and 1061. 

So you ended up bringing in three entire extents with page numbers 1056 - 1063, 1104 - 1111 and 1376 - 1383.

If you do the same experiment against some other SKU than mentioned in the quote (and traceflag 840 is not enabled) you should see your expected three pages (below was against Express LocalDB)