Can't make sense running training load

Hi ,
I’m training for a Marathon so basically only running activities and I have scheduled a plan but the training loads are always underestimated by about 20% .
For example if I simulate one of my activitie already completed :
*0h49m 77% HR (133bpm) → estimated trainning load 40
or *8km 77% pace (6:10) → estimated trainning load 49
Already there’s a difference but then …
Real Run : 8km - 0h49m - Average HR 131bpm - Average pace 6:07 → trainning load 59 .
So if I follow my plan I’m always in the red due to overload trainning.
I’m sure I’ve missed something but would like some help to understand.
Thanks for any help.

Training load by HR will always be less precise. Pace should return better results. That’s because HR has lot’s of lag while pace is instant.
To get the best results, you need to make sure that both planned and done activities use Pace as a primary load metric.
Check your settings and make sure that Training Load and Workout priority are both set to use Pace as a primary metric if you don’t have Power. If one is set to HR and the other to Pace, that would explain the discrepancy you’re seeing.

Well , everything was setup with power as priority … I changed to pace priority and updated my activities and everything gets a lot more in line !
Thanks a lot for your help !!!