Sql roll up doubt how average is calcualted

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SQL-Server

-select * from sales

EmpId	Yr	Sales
1	2005	12000.00
1	2006	18000.00
1	2007	25000.00
2	2005	15000.00
2	2006	6000.00
3	2006	20000.00
3	2007	24000.00

I tried this :

select EMPID,sum(Sales) as [totalsalesemployeewise],AVG(Sales)[avgsalaryemployee] from sales
group by EmpId WITH ROLLUP

i got the result as below :

EMPID	totalsalesemployeewise	avgsalaryemployee
1	55000.00	                   18333.3333
2	21000.00	                   10500.00
3	44000.00	                   22000.00
NULL	120000.00	                   17142.8571

--Now in the last column 120000 is understandable. But how i got this 17142.8571 ?

Kindly help