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Basically, lossy compression codecs store groups of frames, or groups of samples. You just can't "cut" at any frame or sample that you want. If you do, you have to re-encode the whole thing.
If you want to understand why, the "delta encoded" thing means that only the difference between frames (or samples) is stored. It usually takes less bits to store the difference between frames, or samples, than it does to store each frame / sample separately.
The reason why "groups" exist is so that you can jump around in a video / audio file. The beginning of each group stores a complete frame / sample. (This also helps if the file is corrupted.)
As long as you cut a compressed audio / video file at the groups / jumps, you can cut it losslessly.
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(Note: Techniques like companding can use less bits when each frame / sample are stored separately. Some people don't even call this lossless. The technique is basically a digital version of the old "Dolby B" button on tape decks from the 1980s and 1990s.)