Oracle Logical IO / Consistent Reads inconsistent?
My guess would be that the first query doing the extra work of delayed block cleanout because some other session at some earlier point in time modified (or inserted) the data without being able to do a commit cleanout.  That causes Oracle to do a bit of extra work the next time the dirty block is read to update its status.  Here is a good example of the impact of delayed block cleanouts on consistent gets.  I'm not 100% certain that delayed block cleanouts are reflected in LAST_CR_BUFFER_GETS from DBMS_XPLAN but I would certainly expect that they are.