The equation (CCALLS-CCONGS-CMSESTAB) will be zero only if your SD Establishment Rate (not including CNRELCONG) is 100%. Meaning that whenever a MS asks for a SDCCH, it is allocated an SDCCH and the MS also successfully tunes to the SDCCH in the first attempt.
But practically, the MS may be in a low coverage/high interference area where it was unable to LISTEN the SDCCH granted from the BTS so it will resend request for SDCCH (CCALLS). But as the previous granted SDCCH was missed by the MS, (CMESTAB) will not be pegged against that request.
I hope this helps.