I made the test
2 shots using same ISO (200), speed and f stop
Except they weren't the same ISO, because your camera lied.im pretty sure it does sweet FA in raw but i might be wrong.
It just applies a different process to the raw file than it normally would
check the manual
It's not so much that HTP affects the RAW data per se, but it does affect the RAW metadata in a way that's not handled properly by anything but DPP.
What HTP does is deliberately underexpose by one stop, and 'misrecord' the ISO in the metadata - that's why ISO 100 isn't available when you turn on HTP, i.e. you set ISO 200, it shoots at ISO 100 but records 200, or you set ISO 800, it shoots at 400 and records 800. If shooting JPG, it processes the underexposed image to brighten everything except the highlights (meaning it applies a tone curve). If shooting RAW, it sets a metadata flag so DPP can apply that tone curve.
If you open that RAW file in a 3rd party converter, results vary. Some ignore the flag and you just get an underexposed image. Others compensate by just boosting the total exposure by one stop - I think ACR (LR/CS) does that. Of course, that just re-blows your highlights and adds shadow noise. AFAIK, no 3rd party converter tries to replicate the tone curve to preserve highlights.
So, if you shoot RAW and use a converter other than DPP, I'd leave HTP off so your reported ISO reflects the actual ISO used to take the shot, and just expose properly to preserve highlights. You can apply your own tone curve, not limited to the one full stop forced by HTP.
Thanks for the info Neuro, you always amaze me...