Whoever wrote the practice question and the answer may have been using it to address a usage among some communities of speakers of non-standard English, in which “seem” is substituted for standard “seems,” a pattern that is prevalent among certain speakers of English.
However, I do not think that either “seems” or simply “seemed” is correct, and the best phrasing in standard English would have been “had seemed.” The reasons:
The “seeming easy” occurred before the “realizing,” which changed the person's mind. Therefore, at the moment of the realizing, the “seeming easy” was already in the past. But the realizing occurred before the speaker spoke, so he or she put it in the past tense: “I realized.” In standard English, when we refer to one event that occurred before a past event, we use the past perfect tense, which, in this case would read: “I realized it would not be as easy as it had seemed.”
The reason “seems” is not correct (in my opinion) is that at the moment when the speaker had the realization, it no longer seemed easy to him or her. Therefore by using “seems” the speaker would seem to be asserting both that it no longer seemed easy, and yet, that it still “seems” easy, obviously nonsensical as self-contradictory.
Using the simple past “seemed” does not cure the difficulty, because it places the “realizing” and the “seeming” at the same time in the past, leaving the same nonsensical self contradiction; when, in fact, the contradiction disappears if the “seeming” is placed previous to the “realizing,” by the proper use of the past perfect “not as easy as it had seemed.”