[69Y97GP]

Went to buy some new rear springs for my '70 Formula. Eaton Spring shows 2 different spring part numbers for cars with F60-15's. They're both 126 lbs/inch spring rate, but the LEFT has a 5-3/4 inch free arch and the RIGHT is a 5-1/4 inch free arch. Here's the odd part.....they only did this for 70 and 71 on Trans Ams and Formulas (with F60-15s). Then, in '72 they used the 5-3/4 inch free arch on both left and right sides on the Formula, but then yet a different part number for the TA, with a 6-1/8 inch free arch.
So my question is, why the 1/2 inch arch difference between LH and RH for just 70 and 71? And then why did they go to an even larger arch for '72 T/A?

Thanks in advance......