Dear,
I want give a reply to first part of your query, as to why the prospective employers look at the previous CTC while negotiating the future CTC.
Pl. note that the logic behind this practice is that, generally people change jobs for improvement in Financial Status and Unless a person is offered better ctc than the previous one, he may not join.Hence this generally done to motivate motivate a person to leave present job and join new organisation.That way referring present ctc is a thumb rule.
But more important from employers, side is that the position we are trying to fill up and the pay that can be offered for that position,than the previous ctc of the person.Many times as employers we come across good candidates ,but since we can not offer an improvement over existing ctc, and disturb the internal wage structures, we can not recruit them.
Looking from individuals side, yes it is a fact that exiting ctc comes into consideration when we negotiate future ctc, as people may not offer much more that the present ctc . Rarely, we come across cases, where 100% increase over present ctc is offered.