There are a lot of issues here; monitor calibration of the computer as well as the overlay of the graphics card that may be used by the software DVD application. Some software applications also have picture control and some presets for how the picture can look.
Depending on the software playback options, a computer monitor is progressive. If the sources were interlace then there could be some frame blending etc. softening the image.
In addition to monitor calibration there is a difference in black and white level ranges for digital television and graphics and computers. Digital television is 16RGB for black and 235 for white. Computers and graphics use the full 0-255 range.
Then there is the default gamma mode – PC and television use 2.2 where Mac defaults to 1.8.
All of these can have a play in why differences exist.
Michael