sar command output the result at the invalid interval

Issue

• We gather the performance data every 30 seconds for the whole day by sadc.
• The outputs the result by sar command specified with 300 as the interval and 288 as the count.
• But sar command specified with the interval parameter sometimes outputs the result at the invalid interval.
• In particular, sar command output the result at intervals of 300 seconds and 30 seconds even if our customer ran sar command specified with 300 seconds as the interval parameter.

    # LANG=C sar 300 -W -s 00:00:00 -e 00:30:00 -f res_sadc.dat

    Linux 2.6.32-431.40.1.el6.x86_64 (foobar)     08/06/15        _x86_64_        (12 CPU)

    00:00:07     pswpin/s pswpout/s
    00:05:07         0.00      0.00
    00:05:37         0.00      0.00 <- Only 30 seconds has passed form previous output.
    00:10:37         0.00      0.00
    00:15:37         0.00      0.00
    00:16:07         0.00      0.00 <- Only 30 seconds has passed form previous output.
    00:21:07         0.00      0.00
    00:26:07         0.00      0.00
    00:26:37         0.00      0.00 <- Only 30 seconds has passed form previous output.
    Average:         0.00      0.00

• What is the reason why sar command specified with the interval parameter output the result at the invalid interval? How we can resolve this issue?