Have you checked that the inner query is working correctly?
SELECT InterfaceID, [DateTime],
dbo.GetInBps95th(InterfaceID, @StartDate, @EndDate) AS Maxbps_In95,
dbo.GetOutBps95th(InterfaceID, @StartDate, @EndDate) AS Maxbps_Out95,
dbo.GetMaxBps95th(InterfaceID, @StartDate, @EndDate) AS Maxbps_95
FROM InterfaceTraffic
WHERE [DateTime] >= @StartDate
AND [DateTime] <= @EndDate
AND DATEPART(HOUR,[DateTime]) >= 8
AND DATEPART(HOUR, [DateTime]) < 14
For fast, accurate and documented assistance in answering your questions, please read this article.
Understanding and using APPLY, (I) and (II) Paul White
Hidden RBAR: Triangular Joins / The "Numbers" or "Tally" Table: What it is and how it replaces a loop Jeff Moden