I am getting a Tempo instead of Base

Hello all,

Recently (have no clue what triggered this) all my training data just became erratic. 

This morning I did a 8k run with a avg pace of 5:45/km.

My avg HR was 155bpm and Max HR 161 bpm

How do you explain this? 

It gave me a Tempo training effect because I spent 82% of the time in zone 4 - 163-171bpm

However if you have a look at the Lap stats, I've never surpassed 161 bpm

This is driving me crazy ? WTF is going on ?