Something

Guild Wars
Memory locations for health/energy change dynamically (DMA)

.text:004640CA mov     [esi+2Ch], eax    <--- eax contains health value,esi is an memory offset
		       ^_______^ means it is setting a memory location
[esi+2Ch] contains the health value

esi changes per map change, so the memory location where health is stored changes.
The memory location where the value of esi can be found also changes per map change.