Training Effect Aerobic vs Anaerobic

General confusion in my end.

My 5 last runs on treadmill. Same distance/ speed/ machine. 25 mins each.

Run 1 average HR is 166 bpm and max HR is 176 bpm. Aerobic training effect 3.6. Training load 117. Avg resp rate 33 brpm

Run 2 average HR is 164 bpm and max HR is 175 bpm. Aerobic training effect 3.5. Training load 111. Avg resp rate 33 brpm

Run 3 average HR is 161 bpm and max HR is 172 bpm. Aerobic training effect 3.4. Training load 101. Av resp rate 32 brpm

Run 4 average HR is 159 bpm and max HR is 169 bpm. Aerobic training effect 3.2. Training load 97. Avg resp rate 29 bprm

Run 5 average HR is 158 bpm and max HR is 168 bpm. Aerobic training effect 3.2. Training load 100. Avg resp rate 29 brpm

OK first of all not sure why training load went up between 4 and 5. My "basic" wisdom is that if I can do the same thing with lower heart rate I'm making less effort which is a sign of getting fitter (hopefully)..

But the kicker here is the Anaerobic field..

Run 1: 0.3

Run 2: 0.1 

Run 3: 0.0

Run 4: 0.6

Run 5: 1.1

The delta between Run 1, 2 and 3 makes perfect sense. I dipped slightly into anaerobic zone, but small increase in fitness made Run 3 hit 0.0.

But where did 0.6 and 1.1 come from ? How did this suddenly become more intensive ? (with less effort)