Well, why not? Why should anything done on purpose by reasonably thinking creatures not follow any kind of pattern?
More specifically speaking, the Kaiju were created by these alien invaders in order to test our weaknesses, adapt to them and gradually weaken our defenses up to a point where they can start to invade us on a more personal level. With this goal in mind, it is quite reasonable that the attacks get gradually heavier, sending more Kaiju and more often. And given that this follows some kind of alien invasion plan, it is only natural for the aliens to structure it with a strictly computed and prescribed frequency.
The only possible reason for them to bring some more randomness into the attacks, would be for us to not find out the frequency that easily and thus make it harder for us to adapt to the Kaiju attacks (and maybe this is what your question is after). But there could be multiple reasons why this was not done. Maybe the aliens didn't think about this aspect, maybe they underestimated our strength and didn't think that we had any chance against the Kaiju anyway, or maybe they just wanted to give the screenwriter a chance to let the scientists find some pattern in the attacks. Yet, this would be speculation and the latter seems the most likely possibility.