Full Body Training

Rugby season is coming to an end so I intend to return to 5/3/1 full body in the closed season. Last year I ran the program from the second edition but just kept at the phase 1 progression. Beyond 5/3/1 lays out a heavier progression with just 5’s on the non 5/3/1 days. I’m just wondering why you feel it’s a better weight percentage to use? I tried it and felt quite gassed by the time I came to deadlift! A case of manning up and staying NOV!
Thank you in advance