I typically hit my highest heartrates when I am rested, fueled, and weather is HOT and humid.
During the winter my max HR is about 10bpm lower since the heart isn't having to work so hard to cool me off.
Last night I had a 90 minute tough ride. Average HR wasn't that high due to some long regroups and long warmup, but I still saw a lot of time in Z4/Z5.
On this morning's ride I just couldn't get my heartrate up very high and I had my worst ever ascents on two short steep hills I've climbed hundreds of times.
I did what I could to refuel muscles with carbs, but if the heart doesn't want to pump blood as fast as the legs use oxygen, your legs will tire and burn.
Your morning rested heartrate doesn't mean all that much taken as a single reading. The day-to-day trend is what can indicate overreaching/overtraining.