Discrepancy in result when calculating time using decimal(10,1)

I have query that returns total seconds as 28943 (columnname is TotalSec), I am using below query to convert seconds

select ID, cast(todaydate as date) as [Today Date]
, FORMAT(SUM(CAST((CAST(TotalSec as float) / 3600)as decimal(10,1))),'N1')
from @tablename
group by ID,cast(todaydate as date)


output : 1,'07/07/2021',7.6

when I am running below query

select Format(SUM(CAST((CAST(28943 as float) / 3600)as decimal(10,1))),'N1')


output : 8.0

I was wondering why there is discrepancy in output? I tried with below test data

CREATE TABLE dbo.MyTable
(
userID int,
uDateTime datetime,
totalsec bigint
);

GO
INSERT INTO dbo.MyTable VALUES (123, getdate(),28943);
GO
SELECT userid,Format( SUM(CAST((CAST(totalsec as float) / 3600)as decimal(10,1))) ,'N1')
FROM dbo.MyTable
group by userid,cast(udatetime as date)

drop table MyTable


here I am getting output as 8.0

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