Bear in mind this system has a hybrid GPU system - the Intel GPU is ALWAYS active, even when the nVidia GPU is being used - so it can indeed consume RAM -- and Windows since 7 delegates quite a bit to the GPU in all cases, so it is likely completely normal to see what you're seeing.
This isn't Windows 7, and I have two other systems that have an Intel GPU. My Latitude 7480 running Win10 1607 says 16GB with 15.9 usable, and my XPS 15 9530 with a GeForce GPU in addition to integrated graphics running Win10 1709 shows 16GB with no separate note about usable memory at all.
Please re-read before posting. I stated "windows since 7" - not Windows 7.
And the load on the GPU will depend on what's running at any given point in time -- and it WILL vary from system to system and situation to situation.
Strange, both DIMM's separately provided 15,8 GB usable with only one installed. As soon as I installed both again it went back to 25.4GB usable. If this was memory reserved for GPU then I would expect more then 200MB to be reserved with 16GB.
Based on some threads I read in the meantime, in that case it sounds like that matched set may not play well with your system. This post of this particular thread in particular describes somewhat similar behavior: www.experts-exchange.com/.../Installed-memory-RAM-24GB-7-99-GB-usable.html
This one is also somewhat useful: superuser.com/.../why-is-all-my-extra-ram-marked-as-hardware-reserved-in-windows-7
I have added a post on the Crucial support forum for my issue.
Maybe they can confirm that 2 dimm's of this type will not work correctly on a XPS 15 9550
Installed the DIMM's in an other XPS 15 9550 at my employer and that XPS did not have this issue, so this seems to have been a hardware issue with the Dell XPS 15.