about vaccum level

[fegg7502]

Dear Vaspers:

I calculated the vaccum level of my alumina slab with LVTOT = .TRUE., IDIPOL = 3, LDIPOL = .TRUE.,
and then I analyised the LOCPOT file for the Z direction(the vacuum layer is in Z direction). The data are showed below.
the problem is: usually, the vaccum level line in the vaccum area should be a smooth plain line; but there is a shake(it changes from 3.83 to 2.65) in my result..

Could anyone give me some suggestions?
by the way, I checked the results with both 15A and 30A vaccum layer on the Z direction
Thank you very in advance!

-1.06E+01
-1.11E+01
-1.12E+01
-1.07E+01
-9.87E+00
-8.71E+00
-7.39E+00
-6.08E+00
-4.93E+00
-4.13E+00
-3.78E+00
-3.93E+00
-4.58E+00
-5.65E+00
-7.01E+00
-8.44E+00
-9.78E+00
-1.09E+01
-1.16E+01
-1.18E+01
-1.16E+01
-1.09E+01
-9.90E+00
-8.63E+00
-7.27E+00
-6.00E+00
-5.04E+00
-4.51E+00
-4.47E+00
-4.91E+00
-5.82E+00
-7.06E+00
-8.44E+00
-9.77E+00
-1.09E+01
-1.17E+01
-1.20E+01
-1.19E+01
-1.13E+01
-1.03E+01
-9.04E+00
-7.63E+00
-6.26E+00
-5.13E+00
-4.40E+00
-4.16E+00
-4.42E+00
-5.15E+00
-6.25E+00
-7.56E+00
-8.91E+00
-1.01E+01
-1.11E+01
-1.17E+01
-1.18E+01
-1.15E+01
-1.07E+01
-9.62E+00
-8.31E+00
-6.95E+00
-5.71E+00
-4.76E+00
-4.22E+00
-4.19E+00
-4.67E+00
-5.60E+00
-6.85E+00
-8.23E+00
-9.59E+00
-1.08E+01
-1.16E+01
-1.20E+01
-1.19E+01
-1.14E+01
-1.05E+01
-9.32E+00
-7.99E+00
-6.69E+00
-5.62E+00
-4.91E+00
-4.64E+00
-4.85E+00
-5.51E+00
-6.54E+00
-7.77E+00
-9.05E+00
-1.02E+01
-1.11E+01
-1.16E+01
-1.17E+01
-1.13E+01
-1.06E+01
-9.42E+00
-8.08E+00
-6.70E+00
-5.45E+00
-4.48E+00
-3.90E+00
-3.79E+00
-4.16E+00
-4.94E+00
-6.05E+00
-7.37E+00
-8.75E+00
-1.00E+01
-1.11E+01
-1.18E+01
-1.21E+01
-1.19E+01
-1.14E+01
-1.06E+01
-9.64E+00
-8.71E+00
-7.93E+00
-7.39E+00
-7.17E+00
-7.28E+00
-7.66E+00
-8.20E+00
-8.76E+00
-9.21E+00
-9.46E+00
-9.45E+00
-9.15E+00
-8.60E+00
-7.81E+00
-6.87E+00
-5.87E+00
-4.88E+00
-3.97E+00
-3.14E+00
-2.39E+00
-1.70E+00
-1.04E+00
-3.91E-01
2.04E-01
7.31E-01
1.18E+00
1.56E+00
1.87E+00
2.13E+00
2.34E+00
2.52E+00
2.67E+00
2.80E+00
2.92E+00
3.04E+00
3.14E+00
3.25E+00
3.35E+00
3.44E+00
3.52E+00
3.58E+00
3.63E+00
3.66E+00
3.70E+00
3.72E+00
3.74E+00
3.76E+00
3.77E+00
3.78E+00
3.79E+00
3.80E+00
3.81E+00
3.81E+00
3.81E+00
3.82E+00
3.82E+00
3.82E+00
3.82E+00
3.83E+00
3.83E+00
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3.83E+00
3.83E+00
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3.83E+00
3.83E+00
3.83E+00
3.83E+00
3.83E+00
3.83E+00
3.83E+00
3.83E+00
3.79E+00
3.66E+00
3.47E+00
3.24E+00
3.01E+00
2.82E+00
2.70E+00
2.65E+00
2.65E+00
2.65E+00
2.65E+00
2.65E+00
2.65E+00
2.65E+00
2.65E+00
2.65E+00
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2.65E+00
2.65E+00
2.65E+00
2.64E+00
2.64E+00
2.64E+00
2.64E+00
2.64E+00
2.64E+00
2.63E+00
2.63E+00
2.63E+00
2.62E+00
2.61E+00
2.61E+00
2.60E+00
2.58E+00
2.57E+00
2.55E+00
2.52E+00
2.49E+00
2.45E+00
2.40E+00
2.34E+00
2.27E+00
2.17E+00
2.05E+00
1.90E+00
1.71E+00
1.47E+00
1.17E+00
8.11E-01
3.45E-01
-2.95E-01
-1.16E+00
-2.28E+00
-3.64E+00
-5.18E+00
-6.80E+00
-8.35E+00
-9.67E+00

[tlchan]

Are the two surfaces of your slab equivalent to each other?