Think of the tactics trainer as 'pattern recognition' training. For positional understanding it is better to read annotated material like games collections - and this is never going to be annotated. It's just puzzle exercises.
Chess.con Tactics Trainer

On the tactics trainer you win material, it never has anything to do with the position i.e. strategic themes. For example you win a rook, end of puzzle.
Maybe some of them stop before you deliver checkmate so it's confusing what has happened to some players?

Hey, maybe this will help. Each tactic, once completed, shows a list of the themes (pin, defense, etc.) Also, it shows comments below the topic. Everytime I've had a puzzle that baffled me, I've read the comments and somebody explains it perfectly.
Good luck.

Do you know the value of the pieces (as a guide)? i.e. Queen 9 pawns, rook 5, bishop/knight 3. A lot of the trades that are made will be linked to this.
Because I am new to chess, take the following with a grain of salt. Still, I would love to see a small explaination to each tactic. Some, I may hit on and some I may not but that does not mean I understand as to why I made the correct/wrong move. It would be nice to complete the tactic and after the move see a small paragrapgh as to what was lost or gained (position wise) due to the move.
Thought?