Increasing the weight is more difficult for the exercises you’re doing at low weights than at high.
Not just because you’re weaker in those regards and less accustomed to pushing for something harder — though that may be true — but because even a small increment is a much bigger deal. If I’m doing 25 lbs. on something and go up to 27.5, eh, okay, that’s a 10% increase. If I’m doing 5 lbs.* and go up to 7.5? That’s a 50% increase. I would probably not go from 50 lbs. to 75 on a given exercise, or 100 to 150, but at low levels, I don’t have much choice. I’ve already done the part where I add reps and sets; eventually I just have to suck it up, add the weight, and mush on.
“Mush” is more or less what certain upper body muscles feel like right now. 😛 But hey: the next time I add weight on those exercises, it will only be a 33% increase. Which is going to feel like a cakewalk, after this round.
*Why yes, I am utterly lacking** in tricep strength; thanks for asking.
**But I’m getting better!