Matrix builds: mismatched build.number

I have a build config using the Matrix Build feature, which uses %system.build.number% to identify results sent to an external system.

Usually the %system.build.number% for runs of all the generated build configs matches the build number of the top-level build. I've just noticed that that isn't always true, however.

For example, I have a run of the top-level config with build number #439, and the runs of the generated configs are numbered either #439 or #440.

The next run of the top-level config (#440) has all its generated runs numbered #440, so that means some generated configs have two runs numbered #440.

Is that expected behaviour?