Chess Mentor ratings are not comparable to ratings obtained through playing chess--if they were, I'd be a candidate master, which is laughable. Likewise, your Tactics Trainer rating is not comparable to ratings obtained from playing chess against other people.
Think of each rating as being completely separate from the others, and you'll be on the right track.
What are possible reasons my Tactics scores would be half my Chess Mentor scores? Here are obvious ones - being new to chess.com, not enough data points yet, not knowing how the tactics lesson works, and of course needing to learn tactics... The scale of the differences is interesting.