Why is my Intel 160GB X25-M SSD a lot slower than the 80GB one?

I used HDTach to benchmark two SSD drives. One is Intel X25-M G2 160GB and the other is Intel X25-M G2 80GB.

The 80GB has 0.1 ms seek time and 205MB/sec average read while the 160GB has 0.4ms seek time and 68MB/sec average read.

Why is there a big difference between the two drives when only capacity is the difference? Both are running on Windows 7 64bit systems.

Is there anything to do to get better performance from the drives?