When you view the image with a normal image viewer, is it mostly dark? If not, a gamma curve has been applied. That is, does it look like this?
That's what you'll get with a pure dump of linear values into a 16-bit tiff file. Or, encoded across 8 bits, and demosaiced, and with white-balance adjusted, something like this:
If it looks like that or similar, you are likely seeing a linear image. This is not normally considered very useful, so when RAW files are converted to image formats used for viewing and distribution, they're almost always processed into something with a gamma curve applied for that purpose.
I converted the images without changing any color or exposure settings.
You may not have changed settings from the default, but your RAW conversation software does have defaults which are not "leave input values unchanged".
See What does an unprocessed RAW file look like? for more. If you really want to extract unprocessed data for doing your own processing, look at using dcraw as referenced there.