pivot tables

[Fwiz]

i seem to have a average issue with my pivot table, on totals column my total average is not the same as the manual calculation? why is this?

Hour level 1 level 2 level level 4 Grand Total
8 00:02:13 00:14:40 00:00:53 00:02:29 00:04:21 ??
9 00:04:09 00:19:01 00:00:35 00:04:52 00:06:47
10 00:06:19 00:29:09 00:04:30 00:05:23 00:10:16
11 00:03:33 00:15:20 00:00:36 00:04:24 00:05:22
12 00:04:53 00:26:45 00:00:30 00:04:52 00:09:46
13 00:05:44 00:20:36 00:00:23 00:04:37 00:07:35
14 00:02:41 00:20:39 00:00:30 00:04:10 00:08:45
15 00:01:33 00:11:34 00:00:30 00:14:50 00:13:06
16 00:12:38 00:17:05 00:02:21 00:14:22 00:12:46
17 00:13:16 00:06:20 00:00:41 00:07:31 00:08:10

row of data from hour 8
ave
00:02:13 00:14:40 00:00:53 00:02:29 00:05:04 ????

any suggestions?