Stack size is entirely different from amount won.
If you join a table where a player has a $50 stack and you have never seen him play before then his Amount Won will be $0.
If you had 100 hands on this player previously and he'd lost 2 buyins his Amount Won would be -$100.
The amount won is over all hands that you have imported for him not just the current session (but can be affected by filters if these cause some hands to be filtered out).
Tutorial: Configuring HUD Options