Significantly fewer students get a 36 than get a 34 composite score. According to ACT, Inc., there are 25,923 students in the graduating class of 2015 who got a 34 or better composite score on the ACT last year (roughly 1.35% of all test takers). In comparison, there are 10,094 students who got a 35 or better ACT composite score (0.52%) and only 1598 students who got a perfect 36 (0.08%). This means roughly 16 times as many students got a 34 composite score as got a 36 composite score on the ACT.
So while a 34 composite score puts you in the top 99% of test takers, it is still far harder (and therefore more impressive) to get a 36 composite score than it is to get a 34.
But does this difference matter? It depends on what schools you're looking at. If you are looking at the top 2 or 3 schools in the country, the difference between a 34 and 36 could be huge on your application (although no school will reject your application out of hand if they see you "only" got a 34).
On the other hand, a 34 is a top score even for schools with the highest ACT scores. Whether you're applying to an Ivy League school like Yale or Columbia, or an extremely selective non-Ivy like Duke or MIT, a 34 ACT composite score will still place you in squarely in the middle 50% of all students, if not in the top 75% of all students. Rather than taking the ACT again to try and get that elusive 36, you'll be better off trying to strengthen other aspects of your application (like beefing up your personal statement or getting great letters of recommendation from your guidance counselor and teachers who know you well).