Incorrect Sum values the quantity is multiplied by the number of rows
SELECT
info.[OFNumber] 
--Get sum of item quantity from the same category (all quantity before production) PS_ProjectInfo table
,SUM(Case WHEN info.[FK_PSCategory]=2 then ((info.Length * info.[Quantity] )/1000)Else 0 end ) as 'm'
  
-- Get sum of item quantity from the same category (Only quantity produced) PS_DailyProduction table
,SUM(Case WHEN info.[FK_PSCategory]=2 then p.Quantity ELSE 0 END) as 'qty'

FROM 
(
   SELECT [FK_OF], Sum(Quantity)
   FROM [dbo].[PS_DailyProduction]
   GROUP BY [FK_OF] 
) p
Left JOIN PS_ProjectInfo info on info.id=p.[FK_OF]

GROUP BY info.[OFNumber]

BTW, not quite understand why you use left join, since PS_ProjectInfo seems a must