For example, I usually do 21kgs for my triceps, but if I have just started weightlifting that day, I can barely do a single rep or two and I have to rest. After 15 minutes, I can do the same 21kg with ease for several sets and sometimes even with higher weight.
Why can't I do the same 21kgs I've been doing for months now from the moment I start weightlifting?
I experience the same with primarily abdominals, forearms and traps but not with other muscles. Say, I can do the same weight as last time with my biceps as soon as I start weigthlifting.