The game uses this formula
sqrt( ( 2 * Attack ) + Defense + ( Current HP / 2 ) )
to determine ship-strength. However, I don't really know if that does apply to your example here, but it could be - the already tougher weapons of the DL ship would be given much more weight than the HP's of your ships.
Speaking about my own experiences with the DL - if I am not able to overtake their planets ASAP [as in the 'Altaria Resists' tournament] then I am usually going to destroy their ships by integrating a lot of on-type defences. Esp. in your example - when the enemy ship only has 1hp left - this would be a possible way to win, esp. because the defense-tree can be fastly researched.
All you need is a fleet with maxed-out defenses, some minor weapons, and *luck* [plenty of it]. Even a full salvage of a 600ATT won´t degrade all defenses & HP of a fleet stuffed full of ZPA.
And on your turn, you won´t even have to do a lot of damage - if your weapon is offtype the DL defense is 3, +1HP = 4 has to be your min weapon roll - easy.
Just survive the first round.