Well here are my two cents into this subject
It has been proven that Different tribes had somewhat different bows, mainly due to environment issues such as the climate, the terrain, the game etc. How ever, the bows were not made as mass production, but so that they were made to fit a person. In most tribes, clans, villages etc. there was a bowyer (did I spell that right?) or a few who made the bows for the men. In different cultures the bows were different, and they even varied amongst one tribe or village.
The Chiricahua-Apache had a way of measuring the drawlenght of a man so that the man would stretch his arm forward and when it would be straight they would measure the distance from the tip of his thumb to his armpit. This way almost each bow would have a different draw lenght and thus be of different lenght also.
There have been some dominating bow designs in different parts of the globe. For instace most of the bows from the plains of north America were of the gull wing profile. However the Plains tribes used also D bows, recurve bows and gull wing profiled bows with recurves. Some mentions about C profile bows have also been made.
The poundage varied also greatly. Stronger men had stronger bows. Kids bows were light, bows of young warriors were stronger and the bows of fully grown warriors were mainly the strongest. I recall that a Cheyenne man called Strong Left Hand had been mentioned to have a bow significantly stronger than the other men, and that only a few men were able to draw his bow.
So of course the designs vary between persons, but in most cases the bows were of somewhat similar design in certain geographical areas.
I think this was so because the bow was to a primitive man a weapon to feed his family, a weapon to protect and assault. I believe that as nowadays the the people that were truly and deeply interested in bow making and such were a minority amongst primitive people also. So mainly they just wanted a deasent bow to hunt with and that would fit their anatomy.
But then again there are always some unanswered questions such as that why did the Blackfeet Indians have shorter bows in average than the Comanche? Comanches being by average about 165 cm long amongst men, when the same average lenght amongst Blackfeet men was about 182 cm.
These are questions that we can only answered by theories. So Keep introducing different theories