Actually the answer is a mix of all the three.
When you are on the inner-zone of a concentric cell. them some opperators allow you the option to include your own BCCH in the BA list thereby for inter-cell HO the outer-GSM-BCCH is compared with neighbouring BCCHs instead of the inner (DCS) level which is lower than GSM due to propogation loss difference / TX power difference.
The other option being to have a hysterisis defined va parameter to assist you in intercell HOs from inner-Zone. so its good. Pls tell us of your vendor so we can tell you what they are using / which parameter to varify