If I were in place of you, I would assume so. (I am not 100% confident, but if you ask me, I would agree with your inference.)
A similar discussion took place here
Quoting something I found useful-
There may be many reasons for one compiler version being slower than the other. It may depend on some parameters like the load on the judge server at the time of the submission, the data structure that is used in the submission etc.
The user above also mentioned something interesting in the answer, you can have a look (its the topmost answer)
I observed the behavior when I went deeper into the mechanism of gcc, and reading the dump files it produces during compilation, the gimple and control flow graphs that are created all internally. After a lot of research, it was found that different versions might have different optimizations for the same program.