iphone 6s Plus 64GB wrong capacity

It is normal to show a lower number than advertised. The advertised specs are in base 10, I believe, while the lower number is GiB (based on powers of 2 - one GiB is 2^30 = 1,073,741,824 bytes). But 64,000,000,000 ("64 GB") is 59.6 GiB by my calculations, so I'm not sure exactly where the extra 4 GB went. Maybe a separate recovery partition or something.

Yes, usual.

64GB is marketing sugar to help sell the hardware cake. Simply a nice round number that the unwashed masses can use when making buying decisions.

This is an old topic (very old, so I'm surprised it still comes up, especially here) with the same old answer that means reality is never the same when it comes to buying this kind of product.

Savvy devs know this and work their code on the reality, not the marketing. Focus on your apps and let Apple worry about what they tell customers.

As far as I can tell, the device reserves about four to six of those gigabytes for iOS itself.

I mean, I bring up my 32GB phone and it shows in iTunes with a capacity of 27.20 GB with iOS 9.1. But my 32GB iPad 3 shows up as 27.14 GB with 8.4.1 on it. A 32 GB iPad Air shows 26.49 GB with 9.1 on it.

The amount listed as "Other" for space consumed sure doesn't cover what you'd expect an operating installation to cover. The Air currently as that listed as 1.15GB, and I think the iPad 3 had 5 or so GB listed.