Deriving averages correctly in a PIVOT - sql-server

Question

One of our managers has requested a report with a specific format with employee names as rows, Months of the year as columns, and average revenue per month per employee populating the cells. The dates will ultimately be replaced with #startdate and #enddate variables, but for the sake of building and testing I have hard coded one calendar year. To accomplish this request I have created a PIVOT query, however my result set is incorrect and I seldom use PIVOT queries, so I am not knowledgeable enough to understand the problem, and I am hoping for some help from someone who uses them often.
First, here is my query that pulls the correct result set without the requested formatting:
select d.Name, AVG(gross), s.systemmonth, s.SYSTEMYEAR
from desk d, PAYMENTS s
where d.code = s.DESK
and s.ENTERED between '2017-08-01' and '2017-09-30'
group by name, s.SYSTEMMONTH, s.SYSTEMYEAR
order by name
A few rows of the result set:
Name (No column name) systemmonth SYSTEMYEAR
employee 1 221.5737 8 2017
employee 1 181.2476 9 2017
employee 2 161.62 9 2017
employee 2 321.9311 8 2017
employee 3 249.2245 9 2017
employee 4 328.1208 8 2017
employee 4 198.6748 9 2017
employee 5 76.4833 8 2017
employee 5 96.6896 9 2017
Now for my PIVOT query:
select name, [1] AS January, [2] AS February, [3] AS March, [4] AS April,
[5] AS May, [6] AS June, [7] AS July, [8] AS August, [9] AS September, [10]
AS October, [11] AS November, [12] AS December
from
(select d.Name, s.gross, s.systemmonth, s.SYSTEMYEAR
from desk d, PAYMENTS s
where d.code = s.DESK
and s.ENTERED between '2017-01-01' and '2017-12-31'
group by name AS Employee, s.gross, s.SYSTEMMONTH, s.SYSTEMYEAR) a
PIVOT
(AVG(gross)
FOR systemmonth IN
([1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12])
) AS pvt
order by name
The accompanying results, the averages are all incorrect (the NULL is correct however). I've just included 2 months for ease of reading and comparison:
Employee August September
employee 1 279.501 245.478
employee 2 544.9648 235.9713
employee 3 NULL 312.5366
employee 4 403.1505 273.9044
employee 5 129.8883 239.8701
Can anyone tell me why the averages in the PIVOT are not calculating correctly? Thanks for the assistance.

Answer 1

Try removing this line from your pivot query. You are grouping for no reason it seems.
group by name AS Employee, s.gross, s.SYSTEMMONTH, s.SYSTEMYEAR
You could also take your first query and pivot on that and your pivot aggregate would become MAX or MIN at that point because you will only ever have 1 row in the dataset for each employee/month combination.