Lancer vs vampire fight, what am I doing wrong

Hi y'all,

I hope somebody can explane to me where I go wrong in the Incinerator Stations OOP battles. During a lancer vs vampire fight, to me both units should loose their health according the following progress:

Lancer attack = 6, health = 50 and Vampire attack = 4, health = 40 and his vapirism = 50 percents.
Every stike first the vampire looses 6 and then rewins 3 while the Lancer now looses.)

Lancers health : 50    46    42    38    34    30    26    22    18    14    10    6     2
Vampires health: 40 34 37 31 34 28 31 25 28 22 25 19 22 16 19 13 16 10 13  7 10 4  7  1  4 -2
at strike         0     1     2     3     4     5     6     7     8     9    10   11    12

During the 12th strike the Vampires health is reduced to -2 and the Lancer survives with 2 health.
That is, according me but the example says the Lancer should survive with health 14.

Can someone please tell me where I'm going wrong here, it will be a great help?

In advance, thank you.