1. If the reported bug functionality is not covered in FRS, developers treat it is as an enhancement and they may not fix the bug.
2. If they have no time to fix the bug they put it on hold and mention in release notes.
1) The developer think that either the bug has low priority and low severity or not mentioned in the SRS or has not so much impact to the product or prior to release or enhancement or not a justified bug ,and it should be fixed later
2) It may be the case that the bug reported by the tester is not fully understood by the developer and the developer needs some clarification for the bug
1.It may be because of the bug has low priority and the developer is busy in another work(involved in developing another version or involved in fixing high priority bugs)and he might think that he can fix it later when he becomes free.
2.The bug may not be fixed by developer if there will be any changes in the requirements and the development lead may treat it as an unnecessary bug and he tells the developer no need to fix it according to the changed requirements.
1.Due to Hardware n Software limitations
2.May be the coding is correct as per the design documents so that he/she rejected
3.Some times Developer thinks it's correct but tester may not agree with him