Hello Mate,
Any error cause from IMS side (with Bye or cancel message), coming from SBG to the UE, triggering Session abort message on Rx interface (SBG/P-CSCF and PCRF), then PCRF/SAPC will force the EPG/PGW to deactivate the EPS bearer, this will be considered as Normal release by the eNodeB .
for example SBG can trigger Call drop due to RTP Time out (Silent call detected), which will be counted as normal release by the eNodeB.
the ENodeB only counts drops due to Radio link failure or reestablishment failure.
regards