PIC16F88 - compare ADC results with a constant value
You are setting the ADFM bit in ADCON1, which, as your comments indicate, right justifies the result.  This results in the bits being stored as follows:

    (ADRESH)  7 6 5 4 3 2 1 0     (ADRESL)  7 6 5 4 3 2 1 0
   10-bit ADC reading:    9 8               7 6 5 4 3 2 1 0


You then read in the low 8-bits of the result, ADRESL.  You never read in the high byte, ADRESH, containing the top two bits of the result.  Thus you are throwing them away, which are the most signifcant (literally).  When your pot reaches the mid-point, the top bit of the result changs, but you never see it.

Instead, you want to left justify the result, by setting ADFM to 0, and read in just the top 8-bits (ADRESH).  Your test for 128 is now valid.  There is no harm in throwing away the lower 2 bits in ADRESL.

                (ADRESH)  7 6 5 4 3 2 1 0    (ADRESL)  7 6 5 4 3 2 1 0
   10-bit ADC reading:    9 8 7 6 5 4 3 2              1 0