Curiously, Schwalbe make this statement:
Why are tires often narrower than the stated tire size?
In order to ensure that tires have sufficient frame clearance, tire manufacturers generally prefer to keep production closer to the lower end of the permitted tolerance (+/- 3 mm).
Carcass casing materials have become more and more sophisticated over time. That reduces the tire widening after the fitting.
Furthermore, inflation pressure also plays a major role. With maximum inflation pressure the tire becomes wider than with low pressure. And a recently fitted tire still widens over time. This can make a difference of 1 to 2 mm.
The tire width is measured at the widest point, i.e. outside the lug.
(From https://www.schwalbe.com/en/reifenmasse )
This could explain at least half of the difference you're seeing.
Secondly, the internal rim width plays a part in how wide the tyre can be once installed. Again, using Schwalbe's recommendations your 65mm tyre could go on a rim with a width from 21mm to 50mm, which is an enormous difference.
The difference is that the same tyre will be narrower when installed on a rim that is narrower.
I have not been able to find what rim internal width is used for their sizing, but my guess is that your rim is towards the narrower end of their specified range.
How to predict? You can't really, other than knowing about Schwalbe's predisposition toward slightly narrower, and how wide your rims are already.
You might gain a little by increasing the tyre pressure while stored. A tyre will slowly relax, acquire a set and potentially gain a little more width. Adjust to riding pressure before use though.