There is a range size setting named InstanceKeyBlockSize at the BPELConfig:bpal MBean which drives the range of IDs each managed server gets hold of. The default for this is 10000.
When the servers start they pick their ids reading the next block from ID_RANGE table and updates the table for next availabe range.
Say you have 2 nodes , one holds keys from 10000 to 19999 and the other holds from 20000 to 29999 , depending which node you are on you get the next instance id from that pool.
This expalains why one process has an Id starting with 120*** and the other with 110***