I'm not an expert on admissions nor in any inside position but below is my opinion. I'm a student so I do not know what is typical for professors to write on LOR's either:
I think you're getting caught up on that your grade in a particular class will correlate with the strength of the recommendation. What I assume the LOR's should include are personal accounts on dedication and abilities, specificity on studying and research. I have taken classes where I did very well but never spoke to the professor - what is that professor going to say? "He got an 'A' in my class but I never knew his face." I've had other classes where I may not of done as well grade wise but I interacted regularly with the professor. I imagine that those professors are able provide a better LOR based on those interactions. Those were the professors that saw me try hard and can weigh on my abilities to be a good graduate school candidates.
Another thing to consider is some professors will fill the LOR in a matter of minutes while others will spend serious time crafting their opinion. Knowing who will spend time on the LOR to provide a personal account should also play into who you ask. Personally I think more recent is better, but sometimes based on situation it may not be. It may need to be something that should be addressed in your personal statement if your performance degraded latter on. I think admissions staff are used to students having a bad year or two; the importance is to demonstrate that you've overcome whatever challenge that had inhibited you.
