I want to know, does each operator use different different QoS for a default bearer in lte network or it can be same ? If operator chooses either fixed or different QoS then what could be the reason ?
Yes its fixed and most "real" networks still haven't deployed solutions for dynamic QCI updates for default bearers.
QCIs 5 to 9 can be assigned to default bearer. I have seen networks using QCI 7, 8 or 9 as default QCIs.