This is a very valid question, but the short answer (I'll explain more later) is that Curt tested the # C13704 and found that it's bracket design can withstand more weight than the # 13703. This is most likely due to the brackets being smaller and more limited in how they can install and using those install methods it can support more weight safely.
If the # 13703 happens to have enough weight capacity for your e-bike and rack though you'd be perfectly fine to use it.
For an e-bike rack... view full answer...
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