What's the difference in this tire rating

What's the difference between these ratings? I get side wall color, speed rating and load... but why? What makes it different?

I'll probably get the 125q because prefer BSW and speed rating 99 is fine. But I dont get what is different in the tire that gives the other higher speed r and a 1lb difference in weight. Both are 10ply...



Specs details on the link below

 

Wish I could help. Some of the tire stuff I dont think I will ever understand. Good luck in your search, those look like nice tires.

 

Something about the rubber compound gives it a better speed rating. Pretty dang close to the same. And i agree 99mph is plenty good for driving a heavy truck lol

 

Thanks. Just wanted to make sure I wasnt ignoring something key

 

Our trucks are speed limited to 97 MPH anyway so you don't need anything better than that.