How aware are grad school application readers of differing standards internationally?
Repeating a bit other remarks: in the U.S., exams in upper-division courses are usually designed by the individual instructor, and vary widely.

There is a tradition in top-tier places in the U.S. to "challenge" students on exams, and students seem to expect this, while, yes, in some local cultures students expect/demand exams that contain no surprises. 

For that matter, it is not clear to me that "testing cleverness" is necessarily an important goal for timed exams. I am well aware that there is a huge tradition in which quickness and cleverness are construed as fundamental skills/talents. Certainly these are important for math contests, and occasionally useful in the practice of mathematics, but I don't think our classes teach these things, in any case. Thus, testing for quickness and cleverness is testing for something outside the course content. Thus, in many regards it is an irrelevant challenge to the students.

That is, substantial mathematics often has genuine complications and difficulties that exist despite examples and forthrightness, and it is sometimes (often?) counterproductive to create "challenging (surprise) problems" from material that should be straightforward.

But, yes, people on admissions committees are well aware of such traditions and their variations, don't worry.

For me, the letters of recommendation from math faculty are far more important than grades on the transcript, in any case. Similarly, the statement of purpose of the applicant tells much more than the transcript. And, one more time, the skill set relevant to surprise questions on timed exams becomes ever less relevant throughout grad school.