My best guess is that you have a CMYK document, so you are getting CMYK color, and that is the closest equivalent in that color space.
If you turn on either Overprint or Separation Preview the color previews in the document's CMYK space:
You can see that Overprint is turned on in my title bar
InDesign documents don't have a color space—you can have a mix of RGB, CMYK, Lab, and grayscale objects. However, if there's transparency anywhere on the page the document's Transparency Blend Space is used for the preview and flattening on export or output and that can be either RGB or CMYK
CMYK blend space:
RGB Blend Space: