The SC's do seem quite random at times but there is also a strong connection to seniority and SC. I complain about it just like the next guy but if your senior on RSV you shouldn't be doing as many as the junior guys on the average.
Also being on RSV on the weekend tends to mean you WILL be working either via SC or flying as coverage is always much lower.

If your lucky to be in a fat category with a little seniority....well I did 1 SC last month and 1.2 hours of flying. Have less than 30 days before I dequal in a domestic narrowbody.
So next time I forget this and start complaining about my schedule someone remind me that it isn't always bad.