Value of SYSTEM_TIME message inconsistency

I am trying to correct HIGHRES_IMU data with Unix time of each message by comparing to SYSTEM_TIME' message that provides both Unix and time since system boot. However, as a result shown in my record, theSYSTEM_TIME` gave me some wried value as follow:

pixhawk_time_unix_usec	pixhawk_time_boot_ms	current_unix_time
...
1513510367023120	12993735	1513510370041960
1513510367316120	12994030	1513510370334721
1513510367722120	12994435	1513510370853346
1513510367824119	12994539	1513510371072823
1513510368126119	12994841	1513510371260276
1513510368610119	12995346	1513510371987285
1513510369902119	12996649	1513510373057885
1513510370018119	12996755	1513510373164285
1513510370320119	12997059	1513510373580422
1513510370416119	12997155	1513510373681127
1513510370827119	12997564	1513510374301279
1513510371427119	12998167	1513510374998639
1513510371823119	12998561	1513510375507347
1513510371928119	12998666	1513510375606718
7.99867651800583E+018	704532493	1513510376314139
1513510372939119	12999676	1513510376615304
1513510373438119	13000174	1513510377460127
1513510374042119	13000779	1513510378152635
1513510374644119	13001381	1513510378649546
1513510375353119	13002090	1513510379030014
1513510375949119	13002687	1513510379419019
1513510376158119	13002894	1513510379512102
1513510376552119	13003292	1513510379797310
1513510376647119	13003391	1513510379895348
...

the problem is on this line
7.99867651800583E+018 704532493 1513510376314139
I recorded the data for 4 hrs, and this problem shows only one time
how it came and is it another information sending via SYSTEM_TIME message?