It's true that the triceps are involved in shoulder extension which does take place during a deadlift, but it is very indirect. The lats are mostly involved in that contraction.
I would approach it from two other angles:
1. Deadlifts and Squats both involve the lats in stabilization, but I would argue that the lats are definitely more active in the deadlift. The lats play a role as your base for benching, so if the lats are more fatigued from deadlifts as compared to squats, then they would effect the bench more so than squats.
2. Also, I bet grip could be related. I think we've felt the difference between gripping the bar lightly and trying to squeeze the life out of the bar. The same weight on the bar can feel much easier when squeezing the bar hard. However, if your grip is already fatigues from multiple deadlift sets, then your bench would be effected.
James 1:16-17 ESV
Do not be deceived, my beloved brothers. Every good gift and every perfect gift is from above, coming down from the Father of lights
With God's help...Mens sana in corpore sano