This is by design. The numerical values represent true intensity of the R, G, B channels. In RWPaint, for example a value of 40 represents an intensity in the middle of 30 and 50. This is not the case in other editors, because the sRGB and its gamma of 2.2 is not linear. All this complexity is hidden in RWPaint and a value of 50% actually means half-intensity. I would recommend to leave the gamma at 2.2, the setting may even disappear in new versions.
Do you actually need to copy the numerical value between editors? If yes, there is the hexadecimal code below and that follows the sRGB color profile.