Logical Reads vs. Records Returned
Is there a generally accepted correlation between logical reads and the amount of data returned by a query?
  


The missing piece is the

1) The number of logical reads of pages with no relevant data

2) The number of relevant result rows on each page

And these two are determined by your indexes.  If this table has no relevant indexes, then you will have to scan each page, and you will have to read a lot of completely or mostly irrelevant pages.  Even if your index supports a query plan where you read only pages (mostly) full of relevant rows, if those rows are very wide, then there may not be very many (A,B) pairs per page.

If you had an index on (B,A) on this table then SQL could seek to the first row where B=1 and start scanning rows until it hits a row with A>SysUtcDateTime().  This would only require ~4 Logical IOs to get to the first page, and then every Logical IO would result in ~700 (A,B) pairs.