The 2nd one takes into account all failures whatever the reason of SDCCH usage (NA, LU, SMS, DETACH, etc)
Are you giving daily values, or busy hour values ? If daily, then it is normal to have a good LU success : during the night, the number of LU remains stable (periodic LU) and there is no congestion and clean frequencies.
If there is a difference at busy hour, then it is weird.
Now, it seems to me that you are comparing NSS indicators with BSS indicators. I can’t help you here : each system is computing indicators in its own way. Double check the formula and they way each NSS counter is triggered. You might be surprised…?