I have a cheap dvd player $40 bucks and a good one $500 bucks. The cheap one works fine but tends to skip like yours on some cd's whereas it won't on the good one.
The reasons for this is the good one probably has better laser and pickup lasers and less jitter. But a better explaination I think is in error correction. If I remember correct every disc has error correction codes. When a player encounters a error in a disc it searches for the error codes to fill in the blanks. If it can't resolve the error you get a skip. So some cd/dvd players have better alogrithyms than others in error correction.
Just my .02 cents