People tend to think of training zones as hard numbers when in fact there is a lot of overlap. High zone 3 for example, also has some low zone 4 in it. In this digital age we often see things as stepped or absolute when they are actually analog and dynamic. There will always be some portion of your muscles working at different intensities. Also, hr is a lagging indicator. It trails effort and late in rides is often higher than it would be for the same effort earlier in the ride. Just a couple things to consider when heading out for a long ride.