nvarchar vs varchar \ IN vs JOIN - tsql

Question

Question regarding "IN" versus "INNER JOIN" and VARCHAR versus NVARCHAR
Examples lay it out...
This query returns the CORRECT results (11 count):
Uses nvarchar(10) data columns for comparison (Id and Location) via INNER JOIN
SELECT b.Location
FROM [configuration].[dbo].[M3_Customer] a
inner join [configuration].[dbo].[M3_WarehouseInventory] b
on a.Id = b.Location
WHERE 1=1
and b.[WarehouseId] = 'NCL'
AND a.[Status] <> '90'
GROUP BY b.[Location], a.Id
This query returns the CORRECT records (11 count):
Uses the same query above as a sub-query, still comparing the same columns but using IN, and adds a forced CONVERT from NVARCHAR to VARCHAR.
SELECT *
FROM [configuration].[dbo].[M3_Customer]
WHERE 1=1
AND convert(varchar(10),[configuration].[dbo].[M3_Customer].[Id]) in (
SELECT convert(varchar(10),b.Location)
FROM [configuration].[dbo].[M3_Customer] a
inner join [configuration].[dbo].[M3_WarehouseInventory] b
on a.Id = b.Location
WHERE 1=1
and b.[WarehouseId] = 'NCL'
AND a.[Status] <> '90'
GROUP BY b.[Location], a.Id
)
This query returns INCORRECT results (10 count):
Only difference between this query and above is the comparison columns are converted to NVARCHAR
SELECT *
FROM [configuration].[dbo].[M3_Customer]
WHERE 1=1
AND convert(nvarchar(10),[configuration].[dbo].[M3_Customer].[Id]) in (
SELECT convert(nvarchar(10),b.Location)
FROM [configuration].[dbo].[M3_Customer] a
inner join [configuration].[dbo].[M3_WarehouseInventory] b
on a.Id = b.Location
WHERE 1=1
and b.[WarehouseId] = 'NCL'
AND a.[Status] <> '90'
GROUP BY b.[Location], a.Id
)
This is the original query that returned INCORRECT results - 10 records, should be returning 11 records:
The issue is associated to the difference between an INNER JOIN and IN - as well as - varchar vs nvarchar
SELECT *
FROM [configuration].[dbo].[M3_Customer]
WHERE 1=1
AND [configuration].[dbo].[M3_Customer].[Id] IN (
SELECT [configuration].[dbo].[M3_WarehouseInventory].[Location]
FROM [configuration].[dbo].[M3_WarehouseInventory]
WHERE 1=1
and [configuration].[dbo].[M3_WarehouseInventory].[WarehouseId] = 'NCL'
GROUP BY [configuration].[dbo].[M3_WarehouseInventory].[Location]
)
AND [configuration].[dbo].[M3_Customer].[Status] <> '90'
The INNER JOIN logic works with nvarchar.
The IN logic works when converting to varchar.
The IN logic using nvarchar failed to return a single record.
We applied LTRIM(RTRIM() to all columns at one point but that did not resolve the issue.
Only using the combination of IN and converting the comparison columns to varchar - resolved the issue.
Why?