From a quick eyeball it seems as if the discrepancies are all by a factor of 24 hours adrift, maybe look at the underlying numeric vales of your EXTRA_TIME_1 & EXTRA_TIME_2 fields bearing in mind the underlying interval numbers are such that :
1 is 1 day as in 24 hours
0.5 is half a day as in 12 hours
1.5 is a day and a half is 36 hours
A frig might be to only use the frac() values in your calculation, but I suggest caution frigging it like that before understanding the underlying root cause.
frac(EXTRA_TIME_1) + frac(EXTRA_TIME_2)