I think I can help out here a bit.
The 200 GB native backup number means that you will get a maximum of 200 GB on the tape with 0% compression, or basically an exact, unmodified copy between source and tape. The 400 GB compressed number comes from an estimated 2:1 compression ratio of the data. All the tape does is to store the data written from the tape drive. It is the tape drive that contains a hardware compression / decompression algorithm that controls compressing the source data before writing it to tape.
A set of data's ability to be compressed comes not from theoretical numbers of the tape manufacturers but the compressibility of the data, as allided to above. It is best explained with an example of very generic compression techniques. The most basic one I know scans through a block of data of size X and sees if there are repeated sequences of bytes of Y bytes in length. If this sequence is located the compression algorithm removes the later repeated sequence of bytes and replaces them with a pointer to the start of where it first saw the sequence and how many bytes were replaced. See the examples below, hopefully my explaination will be clearer then.
Example 1 - Data string with no repeated data blocks:
1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
Example 2 - Data with some repeated blocks
1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
1234567890abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ
A Z
Now in example 1 you would not get any compression if backed up to tape as there is no sequence of repeated characters.
But in example 2, the 12...YZ line of 62 bytes repeats two times. Say it only takes 6 bytes for the compression algorithm to point the first occurance and note that it removed 62 repeated bytes, then the 124 bytes making up the second and third lines is reduced to 12 bytes. The last line has 60 spaces that can be reduced to 7 bytes, one for the space and 6 for the pointers and count giving backup 53 more bytes. My basic compression here reduces 248 bytes of source data down to 83 bytes. This is approximately a 3:1 compression ration or a 66% reduction in data size between source and tape.
So if you had files that were compressible on average of 3:1, you would see approximately 600 GB of data on your LTO-2 tape.
Files like images and already compressed archives like Zip and Rar files are already compressed so the tape drive does not give much, if any, additional compression to them. Other files like database files with lots of white space for padding out data cells compresses very well.
Hope I cleared this up a bit.
