It's quite simple, I was tired. For each set today I was a few seconds slower than for Week 6 Session 2. Today was back to back with a long (actually my furthest ever) session yesterday evening whereas last time I had 2 rest days in between.
In that background it was OK with the warm-up in under 5:30 and a grand average under 2:30min/100m. My 'fast' 100m were also a few seconds down on last time so my hopes of 5% improvement across 5 weeks have taken a small step backwards.