The remit of this site is not discussion of the motivation of writers but the meaning of their words. We leave the reader to speculate on their motivation.

> 1. ”Something **can** happen” means that it is possible according to the
> laws of physics (or in our experience of the world).
> 
> 2. ”Something **may** happen” means that it is possible according to the
> laws of society (or a social institution).

[See e.g. [Merriam-Webster](https://www.merriam-webster.com/words-at-play/when-to-use-can-and-may)]

So I would interpret this as a combination of (1) a warning, and (2) a “legal” statement:

1. If you do this it is possible that your exam will be returned with a lower grade.
2. If the marker does lower your grade his action will be perfectly legitimate within our regulations.