Im the same exact way bro, I will be busting out heavy weight for 6 reps then BAM 7th rep is almost impossible to get up, my right arm is struggling more than my left arm, and I have to stop at 7. And in my head I was thinking around the 4th and 5th rep "OK fuck yea Im gonna do 10" this is so annoying bro, but I dont ever train more than 6-8 reps, 10 reps if its curls. Everything I do is between 4-8 reps, so my body is conditioned that way.
Also, your body may genetically be more predominant in fast twitch fibers. I know if you work at it, you can convert them to slow twitch, but it sounds like ur like me and it is just way easier to do heavy weight rather than something for 12 reps. IDK what to tell you, I would maybe keep the core lifts to low reps and all the supplementary lifts in the 10-12 rep range and see if it correlates back to your core lifts?