Why does Adobe Illustrator change my web colors?

I opened a new document in Adobe Illustrator CS5, which defaults to CMYK as a color mode. I imported a raster image, used LiveTrace with 6 grayscale colors, expanded, and saved as SVG, which changed the color mode to RGB (which I believe makes no difference, since it's only grayscale colors at this point). In the text, I replaced black in the plain text of the SVG to one web color and gray with a tint of that web color, where the opacity is proportional to the amount of gray. In the example below, I used 7B68EE for MediumSlateBlue. I copied the design in the SVG file with 7B68EE into a CMYK document and the color became 666699.

Here are two images of the difference for this case. The SVG with fill="#7B68EE" in the plain text:

The design pasted into a CMYK document:

A similar replacement happened from Orange (FF8C00) to FF9900 and from LimeGreen (32CD32) to 66CC33. I wonder if it's a coincidence that the hexadecimal characters in Illustrator's colors are doubled, e.g. 323232 becomes 333333.

I am using macOS Mojave 10.14.6 and Illustrator CS5 15.1.0 (yes, I really am, I was able to make them play nicely).

Why does Illustrator change MediumSlateBlue (7B68EE) in an RGB document to 666699 in a CMYK document?

Answer

Hex values are RGB colors. You are essentially asking why an RGB color is not matching in CMYK.

I copied the design in the SVG file with 7B68EE into a CMYK document and the color became 666699.

CMYK colors will often not match RGB colors. If you need CMYK colors, you basically can't use hexadecimal color definitions. Or you need to be prepared to adjust the CMYK using CMYK values, not hex values. Never use any hex value if you are seeking a CMYK color. Start with the hex for "closest possible" then adjust the CMYK values to better match any RGB color.

Possibly helpful, if not a duplicate: Mismatched CMYK Values