40:
A couple of reasons they are 'different' than aftermarket springs of the same rate.
1. Length. 470mm is the oem length, the 400 springs are 530mm or so. The longer a spring is, the more linear movement it will take with a smaller change in rate. Exaggerate to make the point. Take a 3' long spring rated at 20lb/in. and a 3" long spring that is rated at 20lb/in. The shorter spring is going to give you that rate for a much shorter distance of compression than the 3' will.
2. OEM springs are hotwound, aftermarket springs generally coldwound. A hotwound spring will in most instances keep its length longer than a coldwound spring will. That's a good thing if you want your springs to keep their length inbetween oil changes.
3. Cuz they are superior.
I've used .40 aftermarket springs (470mm) and the .40 honda springs. The difference was considerable. A much softer ride on the top of the spring, a more smooth, linear compression.
While I've heard argument to the contrary, I have heard no reasonable argument why a 3.5" steel tube in a fork is better than using that 3.5" for spring length. 'The kawi engineers must've used a 470mm length for some reason' means nothing to me.
In the end, I imagine it amounts to what suits you. The honda springs were recommended by a local suspension tuner. I tried a set due to aftermarket (brand new) spring failure..one of 'em collapsed. The difference was obvious, apparent and a hands down choice.
Whether they suit anyone else is certainly up to the their discrimination.