Recently my supervisor received review request from very reputed journal of Elsevier and he passed it to me because he is not much familiar with the field in which paper was written, but as I was independently doing work in that field so I think he was right passing it to me for review.
So on what basis the editor of journal could have decided to send review request to my supervisor who is not expert in that field ?
I am an Elsevier editor. When I am looking for reviewers, I try the following:
People who I know personally who are doing work relevant to the paper, if I haven't asked them to review another too recently. Occasionally they disagree about its relevance: for example, I asked a colleague who wrote a paper on thermodynamics of coastal ocean sediments to review a paper on thermodynamics of lake sediments. I thought there couldn't be a better match, but he declined because he thought it was too far from his expertise.
People who come up on a Scopus search as having recently published several papers on similar topics. There's a tool built into the editing software to help find them. I usually only check the titles and journals of publication of the listed papers by matched authors to see whether they are in fact a good match, and try to make sure they are first author of the relevant papers.
The author's suggestions. Sometimes these are way off base -- last time I declined a review request that was well outside my field, the editor commented that I had been the author's suggestion (and it was an author I'd never heard of).
Keyword matches in my journal's reviewer database. I use these only if it isn't a highly specialised paper so the matches to fairly generic keywords are likely to be relevant.
Suggestions made by other potential reviewers who have declined review invitations. These can be very helpful.
If I am desperate, I will ask people whose work is relevant to only one aspect of the paper and find other reviewers for other aspects to fill the gaps. For example, I might ask a marine chemist to review a specialised marine biochemistry modelling paper, but also ask a marine modeller to review the modelling aspects.
Your supervisor might have come up in a search if he has co-authored papers with you on the subject. He might also be perfectly well qualified to review it, but think you are even better qualified. If you are a PhD student and he is qualified to supervise you, then I'd suggest he is certainly qualified to review a paper on your topic. If you are a postdoc or higher, this may not be the case.
The authors may have suggested your supervisor, or your supervisor may have previously provided a good review for that editor. It's hard to know what happened without being able to read the editor's mind.
Though you don't state it, I hope your supervisor has approval from the editor for you to do the review. This serves two functions. One, you should receive recognition for doing the review. Two, the editor should know who is doing the review, and they may not want students (this is unlikely).
