a question about relaxation of hex-diamond

[dxcharlary]

I have done a relaxation of hex-diamond in 0GPa. however when I use the structure in CONTCAR in singal point calculation, I find there is a stress of 27.8GPa. I don't know why.
the OUTCAR of relaxation:
...
free energy TOTEN = -78.843385 eV
...
Total -0.00514 -0.00837 0.00341 0.00000 0.00000 0.00000
in kB -0.19609 -0.31953 0.13016 -0.00002 -0.00003 0.00000
external pressure = -0.13 kB Pullay stress = 0.00 kB
...

the OUTCAR of single point calculation:
...
free energy TOTEN = -80.679530 eV
...
Total 7.12213 7.28801 7.30650 0.00000 0.00000 0.00000
in kB 271.93919 278.27272 278.97888 0.00005 0.00007 0.00005
external pressure = 276.40 kB Pullay stress = 0.00 kB
...

here is my INCAR of relaxation:
PREC = Accurate
ENCUT = 1000.0
EDIFF = 1e-6
IBRION = 2
ISIF = 3
NSW = 400
ISMEAR = 0 ; SIGMA = 0.0250
POTIM = 0.020
ISTART = 0
LCHARG = FALSE
LWAVE = FALSE
#Target Pressure
PSTRESS = 0
ISYM = 0
#Crude optimisation
EDIFFG = 1e-4
NSIM=4
LPLANE=.TRUE.
NPAR=2

and there is my POSCAR in relaxtion:
dx C gra
0.529177249
4.1390000000 0 0
0 2.4850000000 0
0 0 4.304000000000
8
Direct
0 0 0
0.3742 0 0
0.5 0.5 -0.16666666667
0.8742 0.5 -0.16666666667
0.8742 0 0.33333333333
0.5 0 0.33333333333
0 0.5 0.5
0.3742 0.5 0.5

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