Oracle has changed the way it derives sessions and processes parameters in database.

In 10g, oracle used to derive sessions parameter from processes parameter using following formula

(1.1 * PROCESSES) + 5

In 11g R1 onwards it changed to

(1.5 * PROCESSES) + 22

It has another rule though:

If we set lower value of sessions parameters than derived value, Oracle will automatically bump it to above derived value.
If we set higher value of sessions parameters than derived value, Oracle will consider our set value

So it always takes which ever is higher

SQL>select name, value from v$spparameter where name in ('sessions','processes');

NAME                   VALUE
------------------------------ ----------
processes               4500
sessions               5000

SQL>select name, value from v$parameter where name in ('sessions','processes');

NAME                   VALUE
------------------------------ ----------
processes               4500
sessions               6776

We can see derived value (4500*1.5)+22=6776 is greater than set value of 5000 in spfile. So its taking derived value for this parameter

Lets change the value in spfile to 7000

SQL> alter system set sessions = 7000 scope=spfile;

System altered.

<< Bounce DB >>
SQL>select name, value from v$spparameter where name in ('sessions','processes');

NAME                   VALUE
------------------------------ ----------
processes               4500
sessions               7000

SQL>select name, value from v$parameter where name in ('sessions','processes');

NAME                   VALUE
------------------------------ ----------
processes               4500
sessions               7000

Now you can see its taking the set value, because set value of more than derived value of 6776.

Hope this helps !!

References:

Sessions parameter in 10.2 – http://docs.oracle.com/cd/E11882_01/server.112/e25513/initparams230.htm#REFRN10197

Sessions parameter in 11.1 – http://docs.oracle.com/cd/B19306_01/server.102/b14237/initparams191.htm#i1133629

Sessions parameter in 11.2 – http://docs.oracle.com/cd/E11882_01/server.112/e25513/initparams230.htm#i1133629